… one in a box dispatched by public transport? (6)
I believe the answer is:
sentry
'one in a box' is the definition.
(sentries may stand in sentry boxes)
'dispatched by public transport?' is the wordplay.
'dispatched' becomes 'sent' (I've seen this before).
'by' says to put 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 "Sentinel, watch" , "Watchperson" , "One who stands guard" , "Look-out, guard" , "Military guard" .)