Irritable about one day giving evidence (9)
I believe the answer is:
testimony
'evidence' is the definition.
(testimony is a kind of evidence)
'irritable about one day' is the wordplay.
'irritable' becomes 'testy' ('testy' can be a synonym of 'irritable'**).
'about' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'day' becomes 'mon'.
'i'+'mon'='imon'
'testy' enclosing 'imon' is 'TESTIMONY'.
'giving' is the link.
(Other definitions for testimony that I've seen before include "It's given under oath" , "Court statement" , "witness statement" , "Evidence, declaration" , "Formal statement" .)