Crossword Genius

Joke repository includes one that's extremely hard-hitting (9)

Ross

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".)

I've seen this clue in The Guardian.
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