Clubs avoid habit-forming E number? (8)
I believe the answer is:
additive
'e number?' is the definition.
(E number is a type of food additive)
'clubs avoid habit-forming' is the wordplay.
'clubs' becomes 'c' (abbreviation used in card games such as bridge).
'avoid' suggests deleting specific letters.
'habit-forming' becomes 'addictive' (something addictive forms a habit).
'addictive' with 'c' removed is 'ADDITIVE'.
(Other definitions for additive that I've seen before include "Supplement" , "Produced by summing" , "Substance put into food" , "ingredient" , "Something joined, to food say" .)