Clear troublemaker is wearing cap (6)
I believe the answer is:
limpid
'clear' is the definition.
('limpid' can be similar in meaning to 'clear')
'troublemaker is wearing cap' is the wordplay.
'troublemaker' becomes 'imp' (I've seen this before).
'is wearing' is an insertion indicator.
'cap' becomes 'lid' (I've seen this before).
'imp' put inside 'lid' is 'LIMPID'.
(Other definitions for limpid that I've seen before include "Crystal clear as water may be" , "Completely clear" , "Translucent" , "Crystal clear, transparent" , "Clear and transparent, water say" .)