That's all right - not any duller than before! (2,6)
I believe the answer is:
no matter
'that's all right' is the definition.
I can't judge whether this defines the answer.
'not any duller than before' is the wordplay.
I cannot quite understand how this works, but
'not' could be 'no' (negative) and 'no' is found within the answer.
'duller' could be 'matter' ('matt' can be similar in meaning to 'dull') and 'matter' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for no matter that I've seen before include "pure energy?" , "It's of little consequence - what a torment" , "What's in a vacuum" , "It's not important - what a torment" , "Irrespective of" .)