An eyesore - seen in nasty environment (4)
I believe the answer is:
stye
'an eyesore' is the definition.
(I've seen this before)
'in nasty environment' is the wordplay.
'in' says the answer is hidden in the clue.
'STYE' is hidden in the letters of 'nasty environment'.
'seen' acts as a link.
(Other definitions for stye that I've seen before include "Ocular inflammation" , "irritation to the viewer" , "swelling by organ" , "Inflammation on eyelid" , "producing eyesore" .)