Blood-sucking worm (5)
I believe the answer is:
leech
'blood-sucking worm' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for leech that I've seen before include "one good at sponging" , "Blood-sucking worm or parasitical person" , "Blood-sucking worm or parasitic person" , "Small worm once used for blood-letting, or a parasitical person" , "Clinging type" .)