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