It’s unimportant, but not more dull? (2,6)
I believe the answer is:
no matter
'it's unimportant' is the definition.
(I've seen this before)
'not more dull?' is the wordplay.
'not' becomes 'no' (negative).
'more dull?' becomes 'matter' ('matt' can be similar in meaning to 'dull').
'no'+'matter'='NO MATTER'
'but' acts as a link.
(Other definitions for no matter that I've seen before include "What about it, it's unimportant" , "Never mind" , "Forget it!" , "pure energy?" , "It's not important - what a torment" .)