Clear troublemaker must wear top! (6)
I believe the answer is:
limpid
'clear' is the definition.
(similar in meaning)
'troublemaker must wear top' is the wordplay.
'troublemaker' becomes 'imp' (little devil).
'must wear' means one lot of letters goes inside another.
'top' becomes 'lid' (lid is a kind of top).
'imp' placed into 'lid' is 'LIMPID'.
(Other definitions for limpid that I've seen before include "Clear (writing, water)" , "Translucent" , "Crystal clear, transparent" , "Clear and bright" , "Clear and transparent, water say" .)