Gateman finally grants admission (6)
I believe the answer is:
sentry
'gateman' is the definition.
'finally grants admission' is the wordplay.
'finally' says to take the final letters.
'admission' becomes 'entry' (I've seen this before).
The final letter of 'grants' is 's'.
's'+'entry'='SENTRY'
(Other definitions for sentry that I've seen before include "Picket" , "Military guard" , "Look-out, guard" , "Sentinel, watch" , "Mil. guard" .)