Witness irritable, if given protection (7)
I believe the answer is:
testify
'witness' is the definition.
(thesaurus)
'irritable if given protection' is the wordplay.
'irritable' becomes 'testy' (testy can mean irritable or annoyed).
'given protection' indicates putting letters inside (inserted letters are protected or held inside).
'testy' going around 'if' is 'TESTIFY'.
(Other definitions for testify that I've seen before include "Bear witness in court" , "sworn" , "Give evidence in court" , "take the stand" , "Affirm (on oath)" .)