Slender in his limitations (4)
I believe the answer is:
slim
'slender' is the definition.
(synonyms)
'in his limitations' is the wordplay.
'in' indicates the answer is hidden within the clue.
'SLIM' is hidden in the letters of 'his limitations'.
(Other definitions for slim that I've seen before include "Take off weight as by dieting" , "Lean" , "insufficient" , "Slender and elegant" , "Very small; WW2 Burma commander" .)