Past one's peak in more ways than one (4,3,4)
I believe the answer is:
over the hill
Here is my best explanation:
'past one's peak' is the definition.
I know nothing about this answer so I can't tell whether this works.
'more ways than one' is the wordplay.
I cannot quite understand how this works, but
'more' could be 'over' (I've seen this before) and 'over' is located in the answer.
'one' could be 'i' (Roman numeral) and 'i' is located in the answer.
This may be the basis of clue (or it may be nonsense).
'in' acts as a link.
(Other definitions for over the hill that I've seen before include "as 20 [OLD MAID]", "Duke of York's men didn't get this far", "walk up and down here?", "Getting on", "past one's prime", "Obscured by the scenery", "Up and down", "Into the next valley", "generally declining" and "Past one's best".)