This is breathlessly assertive! (3)
I believe the answer is:
tis
'this is' is the definition.
('tis' can be a contraction of 'this is')
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for tis that I've seen before include "... the season to be jolly" , "Sequel to "Angela's Ashes"" , "It is shortened" , "For poet, that's" , "Pacific trees" .)