Guard second way in (6)
I believe the answer is:
sentry
'guard' is the definition.
(sentry is a kind of guard)
'second way in' is the wordplay.
'second' becomes 's' (common abbreviation).
'way in' becomes 'entry' (I've seen this before).
's'+'entry'='SENTRY'
(Other definitions for sentry that I've seen before include "Person posted to keep watch for some anticipated event" , "Soldier posted to keep watch" , "Sentinel, watch" , "Watchman" , "Lookout person" .)