Confirmation - or, if inactive, confusion (12)
I believe the answer is:
verification
'confirmation' is the definition.
(I know that confirmation can be written as verification)
'or if inactive confusion' is the wordplay.
'confusion' indicates an anagram.
'or'+'if'+'inactive'='orifinactive'
'orifinactive' anagrammed gives 'VERIFICATION'.
(Other definitions for verification that I've seen before include "Proof of a claim" , "Checking of accuracy" , "checking for truth" .)