Clear young troublemaker concealed by hat (6)
I believe the answer is:
limpid
'clear' is the definition.
('limpid' can be similar in meaning to 'clear')
'young troublemaker concealed by hat' is the wordplay.
'young troublemaker' becomes 'imp' (I've seen this before).
'concealed by' means one lot of letters goes inside another.
'hat' becomes 'lid' ('lid' can be a synonym of 'hat').
'imp' inserted into 'lid' is 'LIMPID'.
(Other definitions for limpid that I've seen before include "Clear, like air or water, say?" , "Crystal-clear" , "Crystal clear like pure water" , "'Of liquid, clear and transparent (6)'" , "Clear (writing, water)" .)