One observing son at door maybe? (6)
I believe the answer is:
sentry
'one observing' is the definition.
(a sentry keeps watch or observes)
'son at door maybe?' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'at' means one lot of letters go next to another.
'door maybe?' becomes 'entry' (door is a type of entrance).
's'+'entry'='SENTRY'
(Other definitions for sentry that I've seen before include "Watchperson" , "Mil. guard" , "Look-out, guard" , "One keeps guard" , "Soldier on guard" .)