That’s all right – not any duller than before! (2,6)
I believe the answer is:
no matter
'that's all right not any' is the definition.
I can't tell whether this defines the answer.
'duller than before' is the wordplay.
'duller' becomes 'matter' ('mat' can be similar in meaning to 'dull').
'than' becomes 'no' (I can't explain this - if you can you should believe this answer much more).
'before' means one lot of letters go next to another.
'matter' put after 'no' is 'NO MATTER'.
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" .)