Prisoner stupid to maintain corruption (13)
I believe the answer is:
contamination
'corruption' is the definition.
(thesaurus)
'prisoner stupid to maintain' is the wordplay.
'prisoner' becomes 'con' (abbreviation for 'convict').
'stupid' is an anagram indicator (in a stupid order).
'to'+'maintain'='tomaintain'
'tomaintain' is an anagram of 'tamination'.
'con'+'tamination'='CONTAMINATION'
(Other definitions for contamination that I've seen before include "Addition of impurities or pollutants" , "Pollution; defilement" .)