Not always brief (3,3)
I believe the answer is:
pro tem
'not' is the definition.
Both the answer and definition are adverbs. Maybe there's an association between them I don't understand?
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for pro tem that I've seen before include "Informally, for the time being" , "For the time being, to the old Romans in short" , "until proper arrangement is made" , "Temporarily" , "not indefinitely extended" .)