Watch dispatched by public transport (6)
I believe the answer is:
sentry
'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).
'sent'+'ry'='SENTRY'
(Other definitions for sentry that I've seen before include "Soldier on guard" , "Soldier posted to keep watch" , "Watchperson" , "One keeps guard" , "One who stands guard" .)