Become septic (6)
I believe the answer is:
fester
'become septic' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for fester that I've seen before include "Rankle" , "Decay" , "Of wound, become septic and filled with pus" , "Suppurate" , "Go septic or cause resentment" .)