Tot needs a leg up (3)
I believe the answer is:
nip
'tot' is the definition.
(thesaurus)
'a leg up' is the wordplay.
'a leg' becomes 'pin' (pin is a kind of leg).
'up' shows that the letters should be reversed in order (in down clue: letters go upwards).
'pin' written backwards gives 'NIP'.
'needs' acts as a link.
(Other definitions for nip that I've seen before include "Quick, sharp, bite" , "Pinch; bite" , "Pinch, maybe in the bud" , "Pinch - tot" , "small amount to drink" .)