Worm hot, cold fish sent back (5)
I believe the answer is:
leech
'worm' is the definition.
(I know that leech is a type of annelid)
'hot cold fish sent back' is the wordplay.
'hot' becomes 'h' (abbreviation eg on taps).
'cold' becomes 'C' (eg on taps).
'fish' becomes 'eel' (eel is a type of fish).
'sent back' says the letters should be written in reverse.
'h'+'c'+'eel'='hceel'
'hceel' written backwards gives 'LEECH'.
(Other definitions for leech that I've seen before include "Blood-sucking parasite" , "Blood-sucking creature" , "Blood-sucking worm or parasitical person" , "scrounger" , "Bloodsucking annelid" .)