Cub needs aid to put on weight (5)
I believe the answer is:
whelp
'cub' is the definition.
(I've seen this before)
'aid to put on weight' is the wordplay.
'aid' becomes 'help' (synonyms).
'to put on' says to put letters next to each other (some letters go on or after others).
'weight' becomes 'w' (short for weight).
'help' after 'w' is 'WHELP'.
'needs' is the link.
(Other definitions for whelp that I've seen before include "Boy!" , "part of litter" , "Produce issue" , "impudent young lad" , "Puppy- young wolf" .)