Watch second access (6)
I believe the answer is:
sentry
'watch' is the definition.
(I know that watch can be written as sentry)
'second access' is the wordplay.
'second' becomes 's' (common abbreviation).
'access' becomes 'entry' (entry is a kind of access).
's'+'entry'='SENTRY'
(Other definitions for sentry that I've seen before include "Soldier on guard" , "One keeps watch" , "Person posted to keep watch for some anticipated event" , "Military lookout" , "One who stands guard" .)