Clarke, the English Open champion in 1999 and 2000, posted his lowest round of the season and matched his closing score from two years ago, when he erased a six-stroke deficit on the final day to win.
'I gave myself a lot of opportunities and made a few of them, but 65 is about as bad as it could have been,' he said. 'I have been waiting for a score to get me up and running.'
Clarke started on the back nine at the Marriott Forest of Arden and notched birdies at the 12th, 14th and 17th holes to go out in 33. He nearly holed his 7-iron shot at the first hole for eagle but instead settled for a tap-in that gave him the first of three straight birdies.
Clarke rang up his seventh birdie of the day at the 466-yard par-4 sixth, where he set up a three-foot putt with a fine approach.
'I like the tournament and like the course,' said Clarke, who has five top-five finishes in six starts at the Forest of Arden layout. 'It is one of my favorite courses we play all year on the tour. I seem to play well here most of the time and look forward to coming back. I missed only two fairways and two or three greens all day.'
Moseley bogeyed the second hole but went on a run with six birdies in a 10-hole stretch. He tied the lead with at the par-5 17th, setting up an eagle with a 230-yard 3-wood shot to within a foot of the hole.
The Aussie captured his lone European Tour title in come-from-behind fashion at the 1999 Heineken Classic in his hometown of Perth. Although he recorded only three top-10 showings over the next two seasons, Moseley has played well of late, shooting a final-round, 9-under 63 for a seventh-place finish two weeks ago in the Volvo PGA Championship at Wentworth Club.
'The 63 at Wentworth opened my eyes to the fact I could shoot a low score,' said the 29-year-old Moseley. 'That was good for my confidence. Three years ago when I first came here I wasn't ready to shoot these low scores. I was comfortable shooting 68 but that was as low as I could go. Now I am comfortable shooting low.'
He went on to fire a 65 over the final 18 last week to tie for 22nd at the British Masters.
Graeme Storm, who won the 1999 British Amateur Championship at Royal County Down, collected seven birdies but double bogeyed the second hole on the way to 5-under-par 67. He finished in third place with fellow Englishmen Mark Roe and Grant Hamerton, Scotland's Gary Orr, Belgium's Didier De Vooght and Alberto Binaghi of Italy.
Justin Rose, chasing a second straight victory on his home soil, carded a 4-under 68 to share ninth place with 10 others, including his British Masters runner-up Ian Poulter.
Defending champion Peter O'Malley was four shots off the pace after a 69.
Full-field scores from the English Open