Certify the account has a favourable balance (8)
I believe the answer is:
accredit
'certify' is the definition.
(accrediting is a kind of certifying)
'account has a favourable balance' is the wordplay.
'account' becomes 'ac' (abbreviation for account).
'has' is a charade indicator (letters next to each other).
'a favourable balance' becomes 'credit' (I've seen this before).
'ac'+'credit'='ACCREDIT'
'the' is the link.
(Other definitions for accredit that I've seen before include "Red cacti (anag)" , "Certify, ascribe" , "CID trace (anag.)" , "Give official recognition to" , "Appoint" .)