… crew too heavy? Not half! (5)
I believe the answer is:
eight
'crew' is the definition.
(I've seen this before)
'too heavy? not half' is the wordplay.
'too heavy?' becomes 'overweight' (I've seen this before).
'not half' means to take half the letters.
'overweight' with half the letters taken is 'EIGHT'.
(Other definitions for eight that I've seen before include "Number (of oarsmen?)" , "4x2" , "What's next?" , "Number etched by a skater?" , "Number of the April rainers" .)