Eyesore small yet nasty (4)
I believe the answer is:
stye
'eyesore' is the definition.
(I've seen this before)
'small yet nasty' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'nasty' indicates anagramming the letters.
'yet' anagrammed gives 'tye'.
's'+'tye'='STYE'
(Other definitions for stye that I've seen before include "Viewer's complaint" , "Eye sore" , "Inflammation around the eye" , "Eyelid infection" , "producing eyesore" .)