Briefly, it's positively assertive (3)
I believe the answer is:
tis
'positively assertive' is the definition.
I can't judge whether this defines the answer.
'briefly it's' is the wordplay.
'it's' anagrams to 'TIS'.
However, I am unsure how this is indicated.
Can you help me to learn more?
(Other definitions for tis that I've seen before include "... the season to be jolly" , "It is shortened" , "McCourt work" , "For poet, that's" , "I confirm the identity of" .)