Singular access for person on watch (6)
I believe the answer is:
sentry
'person on watch' is the definition.
(soldier on guard)
'singular access' is the wordplay.
'singular' becomes 's' (abbreviation sometimes used in dictionaries).
'access' becomes 'entry' (entry is a kind of access).
's'+'entry'='SENTRY'
'for' is the link.
(Other definitions for sentry that I've seen before include "Soldier on guard" , "One keeps guard" , "Sentinel, watch" , "Lookout person" , "Sentinel, look-out" .)