Joke repository includes one that's extremely hard-hitting (9)
I believe the answer is:
punchiest
'extremely hard-hitting' is the definition.
(punchy means hard-hitting or having impact)
'joke repository includes one' is the wordplay.
'joke' becomes 'pun' (pun is a type of joke).
'repository' becomes 'chest' (chest can mean a container or repository).
'includes' indicates putting letters inside.
'one' becomes 'i' (Roman numeral).
'chest' going around 'i' is 'chiest'.
'pun'+'chiest'='PUNCHIEST'
'that's' is the link.
(Another definition for punchiest that I've seen is " Most incisive".)