Ordinary to be in credit for a month (7)
I believe the answer is:
october
'a month' is the definition.
(October is an example)
'ordinary to be in credit' is the wordplay.
'ordinary' becomes 'o' (abbreviation e.g. O-Levels).
'in' is an insertion indicator.
'credit' becomes 'cr'.
'to'+'be'='tobe'
'tobe' put inside 'cr' is 'ctober'.
'o'+'ctober'='OCTOBER'
'for' acts as a link.
(Other definitions for october that I've seen before include "Tenth month" , "Bore cot in autumn" , "21st = Trafalgar Day" , "could be a revolution" , "Pre-Caesarean calendar's eighth month" .)