It is briefly part of treatise (3)
I believe the answer is:
tis
'it is briefly' is the definition.
('tis' is short for 'it is'; 'briefly' indicates a short form)
'part of treatise' is the wordplay.
'part of' indicates a hidden word.
'TIS' is hidden within 'treatise'.
(Other definitions for tis that I've seen before include "McCourt work" , "... the season to be jolly" , "Sequel to "Angela's Ashes"" , "That thing's" , "Pacific trees" .)