Spout in unwarranted fashion (4)
I believe the answer is:
rant
'spout' is the definition.
('rant' can be a synonym of 'spout')
'in unwarranted fashion' is the wordplay.
'in' indicates the answer is hidden within the clue.
'RANT' can be found hidden inside 'unwarranted fashion'.
(Other definitions for rant that I've seen before include "Speak intemperately" , "Shout wildly at length" , "Harangue, rave" , "Bombastically declaim" , "Wild speech" .)