More irritable about providing witness (9)
I believe the answer is:
testifier
'witness' is the definition.
(I know that testifier is a type of witness)
'more irritable about providing' is the wordplay.
'more irritable' becomes 'testier' ('testy' can be a synonym of 'irritable').
'about' means one lot of letters goes inside another.
'providing' becomes 'if' (eg 'providing it's sunny' means 'if it's sunny').
'testier' placed around 'if' is 'TESTIFIER'.
(Other definitions for testifier that I've seen before include "One acknowledging" , "person in court?" , "Witness" , "One who may declare" .)