Deny one plugs hit record at first (8)
I believe the answer is:
disclaim
'deny' is the definition.
(I know that disclaim is a more specific form of the action deny)
'one plugs hit record at first' is the wordplay.
'one' becomes 'i' (Roman numeral).
'plugs' means one lot of letters goes inside another (plug can mean to fill a gap).
'hit' becomes 'lam' (to 'lam' means to hit hard).
'record' becomes 'disc' (eg a CD, compact disc).
'at first' means one lot of letters go next to another.
'i' inserted inside 'lam' is 'laim'.
'laim' put after 'disc' is 'DISCLAIM'.
(Other definitions for disclaim that I've seen before include "Forswear" , "Deny, refuse to acknowledge" , "Deny or relinquish" , "Disown or deny, maybe responsibility" , "Renounce" .)