Charlie avoids habit-forming preservative for one (8)
I believe the answer is:
additive
'preservative for one' is the definition.
(preservatives are a type of food additive)
'charlie avoids habit-forming' is the wordplay.
'charlie' becomes 'c' (phonetic alphabet: alpha, bravo, charlie etc.).
'avoids' indicates named letters should be taken away.
'habit-forming' becomes 'addictive' (something addictive forms a habit).
'addictive' with 'c' taken away is 'ADDITIVE'.
(Other definitions for additive that I've seen before include "Supplement" , "Something extra in food" , "ingredient" , "Produced by summing" , "Tie David up with added substance" .)