Plain the man fell, being past it? (4,3,4)
I believe the answer is:
over the hill
'past it?' is the definition.
(I've seen this before)
'plain the man fell' is the wordplay.
'plain' becomes 'overt' (I've seen this in other clues).
'the man' becomes 'he' (pronoun for a man).
'fell' becomes 'hill' (I've seen this before).
'overt'+'he'+'hill'='OVER-THE-HILL'
'being' acts as a link.
(Other definitions for over the hill that I've seen before include "Past one's best" , "Duke of York's men didn't get this far" , "Into the next valley" , "walk up and down here?" , "generally declining" .)