Favouring a bit of an effort (3)
I believe the answer is:
for
'favouring' is the definition.
(I've seen this before)
'a bit of an effort' 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)).
'FOR' is hidden within 'effort'.
(Other definitions for for that I've seen before include "In support of" , "On the side of" , "On behalf of" , "Favouring" , "In favour of; since" .)