The Liberal Democrat leader said there would be more important issues for him - such as "fairer" taxes - in any coalition negotiations in the event of another hung parliament.
His comments come after Mr Cameron said he would not lead another government after the election next year unless there was a commitment to a referendum before the end of 2017.
While Mr Clegg said he stood by his position that there should not be a referendum unless there was a fresh transfer of powers to Brussels, he indicated that it was not a "die-in-the-ditch" issue.
"I am going to disappoint you by not now drawing great red lines," he told BBC Radio 4's Today programme.
"Are there die-in-the-ditch issues which I have highlighted in the past which I care about, frankly, even more than this? Yes. I think fairer taxes is something I've campaigned for.
"That is something I care about much more than the precise mechanics of when a referendum is held."
Mr Clegg said past history suggested there would be EU treaty changes which would require a referendum.