Salesman isn't commonly going to change decor (7)
I believe the answer is:
repaint
'change decor' is the definition.
(change the paint on the walls)
'salesman isn't commonly' is the wordplay.
'salesman' becomes 'rep' (sales rep).
'isn't commonly' becomes 'ain't' (non-standard version of 'isn't').
'rep'+'aint'='REPAINT'
'going to' is the link.
(Other definitions for repaint that I've seen before include "put on another coat" , "To provide fresh cover" , "put on new coat" .)