Perspires as posterior's carrying weight (6)
I believe the answer is:
sweats
'perspires' is the definition.
('sweat' can be a synonym of 'perspire')
'posterior's carrying weight' is the wordplay.
'posterior's' becomes 'seat's' ('seat' can be a synonym of 'posterior').
'carrying' is an insertion indicator.
'weight' becomes 'w' (short for weight).
'seats' enclosing 'w' is 'SWEATS'.
'as' is the link.
(Other definitions for sweats that I've seen before include "Clothes" , "Perspires" , "old soldiers" , "Works hard" , "Casual dress" .)