Number not in style. What an eyesore! (4)
I believe the answer is:
stye
'what an eyesore' is the definition.
(I've seen this before)
'number not in style' is the wordplay.
I cannot really see how this works, but
'number' could be 'e' (e is a kind of number) and 'e' is located in the answer.
The remaining letters 'sty' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for stye that I've seen before include "Observer's problem" , "Viewer's complaint" , "Eyelid problem" , "Eyelid infection" , "Infected swelling of the eye" .)