A new hair product for sweetheart (5)
I believe the answer is:
angel
'sweetheart' is the definition.
(both are terms of endearment)
'a new hair product' is the wordplay.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'hair product' becomes 'gel' (hair gel).
'a'+'n'+'gel'='ANGEL'
'for' acts as a link.
(Other definitions for angel that I've seen before include "Sponsor" , "somewhere in Islington" , "Word linking cake and fish" , "Old coin; theatre investor" , "Ministering spirit" .)