Cold fish served up before hot sponge (5)
I believe the answer is:
leech
'sponge' is the definition.
('leech' can be a synonym of 'sponge')
'cold fish served up before hot' is the wordplay.
'cold' becomes 'c' (eg on taps).
'fish' becomes 'eel' (eel is a kind of fish**).
'served up' is a reversal indicator.
'before' means one lot of letters go next to another.
'hot' becomes 'h'.
'c'+'eel'='ceel'
'ceel' backwards is 'leec'.
'leec'+'h'='LEECH'
(Other definitions for leech that I've seen before include "scrounger" , "Old medicinal sucker" , "Medicinal bloodsucker" , "Bloodsucker; early doctor" , "Bloodsucking annelid" .)