Decontamination caught scrawny rat (9)
I believe the answer is:
cleansing
'decontamination' is the definition.
'caught scrawny rat' is the wordplay.
'caught' becomes 'c' (cricket abbreviation).
'scrawny' becomes 'lean' (similar in meaning).
'rat' becomes 'sing' (both can mean to inform on someone).
'c'+'lean'+'sing'='CLEANSING'
(Other definitions for cleansing that I've seen before include "Purging" , "Nice slang for making thoroughly free from dirt (9)" .)