A bit of a nasty ear infection (4)
I believe the answer is:
stye
'infection' is the definition.
(stye is a kind of infection)
'a bit of a nasty ear' is the wordplay.
'a bit of' says the answer is hidden in 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 "Seeing problem" , "Viewer's complaint" , "Ocular inflammation" , "irritation to the viewer" , "Ocular affliction" .)