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' indicates the answer is hidden within the clue.
'STYE' is hidden within 'nasty environment'.
'seen' is the 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" .)