Watch dispatched by public transport (6)
I believe the answer is:
Here is my best explanation:
'watch' is the definition.
('sentry' can be a synonym of 'watch')
'dispatched by public transport' is the wordplay.
'dispatched' becomes 'sent' (I've seen this before).
'by' is a charade indicator (letters next to each other).
'public transport' becomes 'ry' (abbreviation for railway).
(Other definitions for sentry that I've seen before include "Lookout person", "Watchperson", "Soldier on guard", "Picket", "One who stands guard", "Watch", "Lookout soldier", "Soldier posted to keep watch", "Watchman" and "Person standing watch".)