I am performing in bad-tempered act of witness (9)
I believe the answer is:
testimony
'act of witness' is the definition.
(a witness gives testimony)
'i am performing in bad-tempered' is the wordplay.
'i am' becomes 'I'm' (contraction).
'performing' becomes 'on' (as in being on stage).
'in' means one lot of letters goes inside another.
'bad-tempered' becomes 'testy' (testy means bad-tempered or irritable).
'im'+'on'='imon'
'imon' going inside 'testy' is 'TESTIMONY'.
(Other definitions for testimony that I've seen before include "Court statement" , "Evidence given in court" , "witness statement" , "It's given under oath" , "Evidence, declaration" .)