Beg for a bit of apple a day (5)
I believe the answer is:
plead
'beg' is the definition.
(I've seen this before)
'a bit of apple a day' is the wordplay.
'a bit of' indicates a hidden word (I've seen 'a bit' mean this (the answer is formed by a bit of the clue)).
'PLEAD' is hidden within 'apple a day'.
'for' is the link.
(Other definitions for plead that I've seen before include "Present an argument in court" , "Request" , "Make an emotional appeal" , "Supplicate" , "Beg or petition" .)