Infection found in crusty eye (4)
I believe the answer is:
stye
'infection' is the definition.
(stye is a kind of infection)
'in crusty eye' is the wordplay.
'in' says the answer is hidden in the clue.
'STYE' is hidden in the letters of 'crusty eye'.
'found' is the link.
(Other definitions for stye that I've seen before include "nasty spot" , "Swelling near the eye" , "Eye sore" , "Eyelid sore" , "irritation to the viewer" .)