Sack contains permit (8)
I believe the answer is:
sanction
'permit' is the definition.
(I know this)
'sack contains' is the wordplay.
'sack' is an anagram indicator (sack can mean destroy or spoil).
'contains' with letters rearranged gives 'SANCTION'.
(Other definitions for sanction that I've seen before include "Authorisation" , "Penalty for breaking law" , "Go-ahead" , "Support" , "Official permission or authority" .)