Flat month in a pattern that's unusual (9)
I believe the answer is:
apartment
'flat' is the definition.
('apartment' can be a synonym of 'flat')
'month in a pattern that's unusual' is the wordplay.
'month' becomes 'm' (standard abbreviation).
'in' means one lot of letters goes inside another.
'that's unusual' indicates anagramming the letters.
'a'+'pattern'='apattern'
'apattern' with letters rearranged gives 'apartent'.
'm' inserted inside 'apartent' is 'APARTMENT'.
(Other definitions for apartment that I've seen before include "quarters" , "Flat (chiefly US)" , "Flat (mainly US)" , "property" , "pad" .)