Crew with heaviness, not weak (5)
I believe the answer is:
eight
'crew' is the definition.
(an eight-person rowing team)
'heaviness not weak' is the wordplay.
'heaviness' becomes 'weight' (how much something weighs).
'not' indicates named letters should be taken away.
'weak' becomes 'w' (physics symbol as in W-boson).
'weight' with 'w' taken out is 'EIGHT'.
'with' acts as a link.
(Other definitions for eight that I've seen before include "Have one over the . . . . . to be slightly drunk" , "Number of the April rainers" , "Chaps having row" , "Cube of two" , "power of two" .)