Easily transported by river on a piece of rough board (8)
I believe the answer is:
portable
'easily transported' is the definition.
(I've seen this before)
'river on a piece of rough board' is the wordplay.
'river' becomes 'po' (Po is an example).
'on' says to put letters next to each other.
'a piece of' suggests taking the first letters.
'board' becomes 'table' ('table' can be a synonym of 'board'**).
The first letter of 'rough' is 'r'.
'po'+'r'+'table'='PORTABLE'
'by' is the link.
(Other definitions for portable that I've seen before include "Light enough to carry" , "Handy - lightweight" , "desirable type of pension" , "to take with you" , "Easily transported" .)