Worst year to catch infection (4)
I believe the answer is:
stye
'infection' is the definition.
(stye is a kind of infection)
'worst year to catch' is the wordplay.
'to catch' says the answer is hidden in the clue (the answer is caught in the letters of the clue).
'STYE' can be found hidden inside 'worst year'.
(Other definitions for stye that I've seen before include "Inflammation around the eye" , "Eye blemish" , "makes viewer sore" , "Eye complaint" , "Red eye" .)