The little devil breaks cover and gets clear (6)
I believe the answer is:
limpid
'clear' is the definition.
(similar in meaning)
'the little devil breaks cover' is the wordplay.
'the little devil' becomes 'imp' (an imp is a little demon).
'breaks' means one lot of letters goes inside another (inserted letters break into the word).
'cover' becomes 'lid' (lid is a kind of cover).
'imp' going into 'lid' is 'LIMPID'.
'and gets' is the link.
(Other definitions for limpid that I've seen before include "Clear and bright" , "Clear - tranquil" , "Crystal clear as water may be" , "Crystal clear, transparent" , "Clear, like air or water, say?" .)