Just as brief? Not these days! (2,6)
I believe the answer is:
no longer
'just' is the definition.
Both the definition and answer are adverbs. Maybe you can see a link between them that I don't see?
'brief? not these days' is the wordplay.
'brief?' means to remove the last letter (a shorter or briefer version of the word).
'these days' becomes 'longer' (I am not sure about this - if you are sure you should believe this answer much more).
'not' with its last letter taken away is 'no'.
'no'+'longer'='NO LONGER'
'as' is the link.
Can you help me to learn more?
(Other definitions for no longer that I've seen before include "Not as formerly" , "a person without yearnings?" , "One not disposed to pining" , "Someone who doesn't dream" .)