A bit of nasty ear infection (4)
I believe the answer is:
stye
'infection' is the definition.
(stye is a kind of infection)
'a bit of nasty ear' is the wordplay.
'a bit of' indicates the answer is hidden within the clue (I've seen 'a bit' mean this (the answer is formed by a bit of the clue)).
'STYE' is hidden within 'nasty ear'.
(Other definitions for stye that I've seen before include "Inflamed swelling on the edge of an eyelid" , "Tamworth residence" , "producing eyesore" , "Eyelid affliction" , "nasty spot" .)