Admission of one short of time made an impact (9)
I believe the answer is:
impressed
'admission of one short of time made an impact' is the definition.
I can't judge whether this definition defines the answer.
'admission of one short of time made' is the wordplay.
'admission of one short of time' becomes 'im' (I can't explain this - if you can you should believe this answer much more).
'made' becomes 'pressed' (pressing is a kind of making).
'im'+'pressed'='IMPRESSED'
(Other definitions for impressed that I've seen before include "printed word?" , "Forced into service - unlikely to forget it!" , "Produced admiration or respect in someone" , "Drafted in" , "Markedly affected" .)