"It's hard to walk away now but I think it's the right time.
"As a captain I certainly haven't led from the front."
Clarke broke down during the post-match ceremony, in which he confirmed his retirement.
Steve Smith was already going to captain the one-day team in the upcoming series against England.
Smith will lead the side in their next Test assignment, a tour of Bangladesh in October.
He captained Australia in three Tests against India last summer, when Clarke underwent hamstring surgery.
Clarke returned during the World Cup, but retired from limited-overs cricket after top-scoring in the final.
The 34-year-old has struggled in the whites since, with his current series average standing at 16.71.
Only one other Australian captain has produced a lower average in an Ashes since 1965 - Ricky Ponting's 16.14 in 2010-11.
However, the skipper declared before the fourth Test he had no intention to retire after the Ashes and remained confident the wheel would turn soon.
Coach Darren Lehmann vowed to give Clarke "as long as he needs" to find form after a crushing defeat last week at Edgbaston.
The conversation moved quickly at Trent Bridge, where the tourists effectively ceded the urn when they were skittled for just 60 in the fastest first innings in Test history.
In Clarke's past 30 Test innings, he has reached 25 just six times and scored two hundreds.
The current series will be the fifth time Clarke has played in an unsuccessful Ashes campaign.
The Nottingham nightmare is Clarke's 13th away defeat as Test captain.
No Australia skipper has lost as many away Tests.
Yet there were plenty of highlights in Clarke's career, most notably an unbeaten knock of 329 against India in 2012 at the SCG.
If Clarke fails to reach three figures in the upcoming dead rubber, he will finish on 28 Test centuries - one less than Don Bradman.
Australia will play two Tests in Bangladesh in October then host New Zealand and the West Indies in three-Test series.
- AAP
