Slimmer (6)
I believe the answer is:
dieter
'slimmer' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for dieter that I've seen before include "The weight-watcher" , "One restricting food" , "who wants to lose 13 [BULK]" , "who's getting slimmer?" , "One who's slimming" .)