Slave to a habit (6)
I believe the answer is:
addict
'slave to a habit' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for addict that I've seen before include "Person compulsively given to taking drugs" , "Junkie, substance abuser" , "Junkie, dependant" , "Person hooked on drugs" , "Bricklayer's tool" .)