Month into rota, changed style (3,4)
I believe the answer is:
art deco
'style' is the definition.
(I've seen this before)
'month into rota changed' is the wordplay.
'month' becomes 'dec' (Dec is an example).
'into' indicates putting letters inside.
'changed' is an anagram indicator.
'rota' is an anagram of 'arto'.
'dec' put within 'arto' is 'ART DECO'.
(Other definitions for art deco that I've seen before include "Early C 20 style" , "Old style" , "nineteen thirty's style" , "Style popular in the 1920s and '30s" , "1920s and 30s style of art and architecture" .)