Sanction care of people holding fast (12)
I believe the answer is:
'sanction' is the definition.
'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'.
(Other definitions for confirmation that I've seen before include "religious rite" , "Proof; a religious ceremony" , "Christian rite - additional proof" , "Endorsement" .)