Rowing crew's heaviness cut lead (5)
I believe the answer is:
eight
'rowing crew's' is the definition.
'heaviness cut lead' is the wordplay.
'heaviness' becomes 'weight' (I've seen this before).
'cut lead' means to remove the first letter (the letter which leads the word is cut out).
'weight' with its first letter removed is 'EIGHT'.
(Other definitions for eight that I've seen before include "Number (with pieces?)" , "Half a quarter" , "A number, sounds like swallowed" , "Number of the April rainers" , "Rowing boat crew" .)