Swear whenever irritable drinking (7)
I believe the answer is:
testify
'swear' is the definition.
(swearing or testifying something is true)
'whenever irritable drinking' is the wordplay.
'whenever' becomes 'if' (if or when something happens).
'irritable' becomes 'testy' (testy can mean irritable or annoyed).
'drinking' means one lot of letters goes inside another (inserted letters are consumed or drunk).
'if' going into 'testy' is 'TESTIFY'.
(Other definitions for testify that I've seen before include "sworn" , "take the stand" , "Affirm (on oath)" , "Give evidence in court" , "Bear witness in court" .)