Examiner is more irritable without one (6)
I believe the answer is:
tester
'examiner' is the definition.
(I know that examiner can be written as tester)
'more irritable without one' is the wordplay.
'more irritable' becomes 'testier' ('testy' can be a synonym of 'irritable').
'without' indicates named letters should be taken away.
'one' becomes 'i' (Roman numeral).
'testier' with 'i' taken away is 'TESTER'.
'is' acts as a link.
(Other definitions for tester that I've seen before include "Street (anag.)" , "covering at night" , "one experimenting" , "Spray" , "One examining" .)