Suppurate (6)
I believe the answer is:
fester
'suppurate' is the definition.
(I know that suppurate can be written as fester)
This is all the clue.
(Other definitions for fester that I've seen before include "strange uncle" , "Ripen and generate pus" , "Go septic or cause resentment" , "Of wound, form yellow pus" , "Decay" .)