O to be in credit for a month! (7)
I believe the answer is:
october
'month' is the definition.
(October is a month of the year)
'o to be in credit' is the wordplay.
'in' is an insertion indicator.
'credit' becomes 'cr' (abbreviation).
'to'+'be'='tobe'
'tobe' placed inside 'cr' is 'ctober'.
'o'+'ctober'='OCTOBER'
'for a' is the link.
(Other definitions for october that I've seen before include "Tenth month" , "Month of nineteen seventeen Bolshevik revolution in Russia" , "some weeks" , "It was the eighth month to the Romans" , "To be orc (anag)" .)