Poles originally transported in river vessel (5)
I believe the answer is:
posts
'poles' is the definition.
'originally transported in river vessel' is the wordplay.
'originally' says to take the initial letters.
'in' is an insertion indicator.
'river' becomes 'po' (Po is an example).
'vessel' becomes 'ss'.
The first letter of 'transported' is 't'.
'po'+'ss'='poss'
't' going into 'poss' is 'POSTS'.
(Other definitions for posts that I've seen before include "Stakes" , "a couple seen propping up the bar?" , "Jobs and poles" , "Sends by mail" , "Positions, for guards say" .)