Weight having hit maturity (8)
I believe the answer is:
poundage
'weight' is the definition.
(I have seen 'The weight' mean 'poundage' so perhaps 'weight' could also mean 'poundage')
'hit maturity' is the wordplay.
'hit' becomes 'pound' (pounding is a kind of hitting).
'maturity' becomes 'age' (I've seen this before).
'pound'+'age'='POUNDAGE'
'having' is the link.
(Other definitions for poundage that I've seen before include "Tax by weight - due on gap (anag)" , "Commission; excessive body weight" .)