Lose weight within cautious limits (4)
I believe the answer is:
slim
'lose weight' is the definition.
(synonyms)
'within cautious limits' is the wordplay.
'within' says the answer is hidden in the clue.
'SLIM' is hidden in the letters of 'cautious limits'.
(Other definitions for slim that I've seen before include "Thin (person); small (chance)" , "without much body" , "Elegantly thin or slender" , "Take off weight as by dieting" , "Slenderly built" .)