Irritable if brought in to give evidence (7)
I believe the answer is:
testify
'give evidence' is the definition.
(thesaurus)
'irritable if brought in' is the wordplay.
'irritable' becomes 'testy' ('testy' can be a synonym of 'irritable').
'brought in' means one lot of letters goes inside another.
'testy' placed around 'if' is 'TESTIFY'.
'to' is the link.
(Other definitions for testify that I've seen before include "sworn" , "Give evidence in court" , "take the stand" , "Affirm (on oath)" , "Bear witness in court" .)