Just what a smoker needs: Henry to go off the rails (6)
I believe the answer is:
astray
'off the rails' is the definition.
(off the rails can mean astray or lost)
'just what a smoker needs henry to go' is the wordplay.
'just what a smoker needs' becomes 'ashtray' (a smoker might use an ashtray).
'henry' becomes 'H' (symbol for the unit of electrical inductance).
'to go' suggests deleting specific letters.
'ashtray' with 'h' taken out is 'ASTRAY'.
(Other definitions for astray that I've seen before include "Wandering off the path" , "Off one's route or lost" , "wrong way" , "Going wrong" , "(Gone) off course" .)