It's unimportant, gossip about old medium (2,6)
I believe the answer is:
no matter
'it's unimportant' is the definition.
(I've seen this before)
'gossip about old medium' is the wordplay.
'gossip' becomes 'natter' ('natter' can be a synonym of 'gossip'**).
'about' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'medium' becomes 'm' (as in UK clothing sizes).
'o'+'m'='om'
'natter' going around 'om' is 'NO MATTER'.
(Other definitions for no matter that I've seen before include "Irrespective of" , "It's of little consequence - what a torment" , "Forget it!" , "Regardless of" , "It makes no difference" .)