Cold fish served up with hot worm (5)
I believe the answer is:
leech
'worm' is the definition.
(I know that leech is a type of segmented worm)
'cold fish served up with 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 (in a down clue, going upwards).
'with' is a charade indicator (letters next to each other).
'hot' becomes 'h' (abbreviation eg on taps).
'c'+'eel'='ceel'
'ceel' back-to-front is 'leec'.
'leec'+'h'='LEECH'
(Other definitions for leech that I've seen before include "Archaic word for physician" , "One who sponges on others" , "Annelid worm, used in medicine of old" , "one good at sponging" , "Parasite - hanger-on" .)