It's in a nasty eyesore! (4)
I believe the answer is:
stye
'eyesore' is the definition.
(I've seen this before)
'it's in a nasty' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'in' is an insertion indicator.
'a nasty' becomes 'sye' (I can't justify this - if you can you should give a lot more credence to this answer).
't' placed within 'sye' is 'STYE'.
Can you help me to learn more?
(Other definitions for stye that I've seen before include "Swelling near the eye" , "Inflamed swelling on the edge of an eyelid" , "Viewer's complaint" , "Eyelid sore" , "Ocular affliction" .)