Supposing irritable hosts swear (7)
I believe the answer is:
testify
'swear' is the definition.
(swearing or testifying something is true)
'supposing irritable hosts' is the wordplay.
'supposing' becomes 'if' (I can't justify this - if you can you should believe this answer much more).
'irritable' becomes 'testy' (testy can mean irritable or annoyed).
'hosts' indicates putting letters inside (inserted letters are welcomed or hosted).
'if' put into 'testy' is 'TESTIFY'.
Can you help me to learn more?
(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" .)