Oscar has to be in credit for a month (7)
I believe the answer is:
october
'a month' is the definition.
(October is a month of the year)
'oscar has to be in credit' is the wordplay.
'oscar' becomes 'O' (phonetic alphabet: alpha, bravo, charlie etc.).
'has' is a charade indicator (letters next to each other).
'in' indicates putting letters inside.
'credit' becomes 'cr'.
'to'+'be'='tobe'
'tobe' inserted into 'cr' is 'ctober'.
'o'+'ctober'='OCTOBER'
'for' acts as a link.
(Other definitions for october that I've seen before include "Month of nineteen seventeen Bolshevik revolution in Russia" , "Bore cot in autumn" , "could be a revolution" , "21st = Trafalgar Day" , "Cot robe for this month" .)