Caught by specialist, yeast infection (4)
I believe the answer is:
stye
'infection' is the definition.
(stye is a kind of infection)
'caught by specialist yeast' is the wordplay.
'caught by' indicates the answer is hidden within the clue.
'STYE' can be found hidden inside 'specialist yeast'.
(Other definitions for stye that I've seen before include "Ocular affliction" , "swelling by organ" , "Infected swelling of the eye" , "Eyelid sore" , "producing eyesore" .)