Not entirely light batter (3)
I believe the answer is:
lam
'batter' is the definition.
(I've seen this before)
'not entirely light' is the wordplay.
'not entirely' means to remove the last letter.
'light' becomes 'lamp' (lamp is a kind of light).
'lamp' with its final letter taken off is 'LAM'.
(Other definitions for lam that I've seen before include "Hit hard, strike" , "US criminals commonly escape?" , "Hit hard or repeatedly" , "Thump; leg it" , "Beat hard" .)