The deputy prime minister said he had put "country before party".
Taking his turn before the BBC's election Question Time audience, Mr Clegg was repeatedly challenged on whether voters could trust his party.
He also warned of the influence on government of the SNP or the "swivel-eyed brigade" of Conservative MPs.
The leaders of the three largest Westminster parties appeared separately at Leeds Town Hall.
Main pledges
Policy guide: Where the parties stand
UKIP leader Nigel Farage, the SNP's Nicola Sturgeon and Plaid Cymru's Leanne Wood were also questioned in separate programmes.
Questions to Mr Clegg opened with tuition fees, which his party had pledged to scrap before the last election. The Lib Dem leader said he had been left "between a rock and a hard place" on the issue.
He was also grilled by an audience member who said he had voted Lib Dem in 2010 but would have preferred a coalition with Labour.
"There's just the little matter of democracy," Mr Clegg replied, saying that - because of the election results - the only way to provide a stable government had been with the Conservatives.
He added: "I will never apologise... whatever the short-term political effects on the Lib Dems, for having stepped up to the plate in a very plucky and brave way to put the country before party."
Attempting to position his party as the centre-ground "anchor" of a future coalition, he said his "great fear" was "David Cameron dancing to the tune of Nigel Farage or the swivel-eyed brigade of the Conservative Party, or you have, basically, Ed Miliband at the beck and call of Alex Salmond".
Mr Clegg was also asked about his policy on Trident, the UK's submarine-based nuclear weapons system. The Conservatives are committed to building four new nuclear missile-armed submarines.
Arguing that three submarines would be sufficient to protect the country, he said: "We can step down the nuclear ladder while keeping ourselves safe."
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Lib Dem leader Nick Clegg has said he will never apologise for his "brave and plucky" decision to enter coalition with the Conservatives.