Nasty bug lying concealed (4)
I believe the answer is:
ugly
'nasty' is the definition.
(I've seen this in another clue)
'bug lying concealed' is the wordplay.
'concealed' says the answer is hidden in the clue.
'UGLY' is hidden within 'bug lying'.
(Other definitions for ugly that I've seen before include "Not nice-looking" , "offensive to look at" , "Offensive to one's sight or other senses" , "Not beautiful, plain" , "Lacking in beauty" .)