Clear horror breaking cover (6)
I believe the answer is:
limpid
'clear' is the definition.
(similar in meaning)
'horror breaking cover' is the wordplay.
'horror' becomes 'imp' (both can mean a misbehaving child).
'breaking' means one lot of letters goes inside another.
'cover' becomes 'lid' (lid is a kind of cover).
'imp' inserted into 'lid' is 'LIMPID'.
(Other definitions for limpid that I've seen before include "Crystal clear as water may be" , "Crystal clear, transparent" , "Clear - tranquil" , "Clear and transparent, water say" , "Clear and bright" .)