Slimmer put off eating one (6)
I believe the answer is:
dieter
'slimmer' is the definition.
(I've seen this before)
'put off eating one' is the wordplay.
'put off' becomes 'deter' (both can mean to discourage).
'eating' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'deter' placed around 'i' is 'DIETER'.
(Other definitions for dieter that I've seen before include "who wants to lose 13 [BULK]" , "person who's determined to lose" , "One watching one's weight" , "German boy" , "one trying to lose weight" .)