Hack's first letter goes on record (4)
I believe the answer is:
ever
'on record' is the definition.
(eg the best ever, best on record)
'hack's first letter goes' is the wordplay.
'hack' becomes 'sever' (both can mean to chop).
'first letter goes' means to remove the first letter.
'sever' with its first letter removed is 'EVER'.
(Other definitions for ever that I've seen before include "Possibly" , "At all times, always" , "constantly" , "Always, at any time" , "Continuously" .)