Chat about decoration that's unimportant (2,6)
I believe the answer is:
no matter
'that's unimportant' is the definition.
(if something is no matter it's unimportant)
'chat about decoration' is the wordplay.
'chat' becomes 'natter' (synonyms).
'about' indicates putting letters inside.
'decoration' becomes 'om' (I've seen this in other clues).
'natter' going around 'om' is 'NO MATTER'.
(Other definitions for no matter that I've seen before include "That is trivial" , "Regardless of" , "Irrespective of" , "It doesn't make any difference" , "Don't mind" .)