Slimmer is put off swallowing one (6)
I believe the answer is:
dieter
'slimmer' is the definition.
(I've seen this before)
'put off swallowing one' is the wordplay.
'put off' becomes 'deter' (both can mean to discourage).
'swallowing' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'deter' going around 'i' is 'DIETER'.
'is' is the link.
(Other definitions for dieter that I've seen before include "bloke obeying regime" , "One restricting food" , "who wants to lose 13 [BULK]" , "one trying to lose weight" , "The weight-watcher" .)