Witness may be more irritable about Formula 1 (9)
I believe the answer is:
testifier
'witness' is the definition.
(testifier is a kind of witness)
'more irritable about formula 1' is the wordplay.
'more irritable' becomes 'testier' ('testy' can be a synonym of 'irritable').
'about' means one lot of letters goes inside another.
'formula 1' becomes 'fi' (resembles F1).
'testier' placed around 'fi' is 'TESTIFIER'.
'may be' is the link.
(Other definitions for testifier that I've seen before include "person in court?" , "One acknowledging" , "Witness" , "One who may declare" .)