O to be in credit for one month! (7)
I believe the answer is:
october
'one month' is the definition.
(October is an example)
'o to be in credit' is the wordplay.
'in' is an insertion indicator.
'credit' becomes 'cr'.
'to'+'be'='tobe'
'tobe' placed within 'cr' is 'ctober'.
'o'+'ctober'='OCTOBER'
'for' is the link.
(Other definitions for october that I've seen before include "21st = Trafalgar Day" , "Month of nineteen seventeen Bolshevik revolution in Russia" , "Russian Revolution month" , "Pre-Caesarean calendar's eighth month" , "The Hunt for Red . . . . . . . (film)" .)