Sanction care of people holding fast (12)
I believe the answer is:
confirmation
'sanction' is the definition.
(thesaurus)
'care of people holding fast' is the wordplay.
'care of' becomes 'co' (c/o can mean 'care of' when addressing mail).
'people' becomes 'nation'.
'holding' means one lot of letters goes inside another.
'fast' becomes 'firm' (synonyms).
'nation' enclosing 'firm' is 'nfirmation'.
'co'+'nfirmation'='CONFIRMATION'
(Other definitions for confirmation that I've seen before include "religious rite" , "Proof; a religious ceremony" , "Christian rite - additional proof" , "Endorsement" .)