Things smokers use to protect new teeth (5)
I believe the answer is:
fangs
'teeth' is the definition.
(fang is a kind of tooth)
'things smokers use to protect new' is the wordplay.
'things smokers use' becomes 'fags' (fag can informally mean a cigarette).
'to protect' is an insertion indicator.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'fags' enclosing 'n' is 'FANGS'.
(Other definitions for fangs that I've seen before include "Large pointed teeth" , "Teeth of, say, wolf" .)