Festering group in charge retaining power (6)
I believe the answer is:
septic
'festering' is the definition.
('septic' can be similar in meaning to 'festering')
'group in charge retaining power' is the wordplay.
'group' becomes 'set' (set is a kind of group).
'in charge' becomes 'ic'.
'retaining' is an insertion indicator.
'power' becomes 'p'.
'set'+'ic'='setic'
'setic' placed around 'p' is 'SEPTIC'.
(Other definitions for septic that I've seen before include "Caused by putrefaction" , "Putrid" , "Of wound, forming pus" , "Of wound, infected and pus-filled" , "Contaminated with disease-causing bacteria" .)