A Roman senator originally abandoned one on the river (7)
I believe the answer is:
oarsman
'one on the river' is the definition.
(someone rowing in a river)
'a roman senator originally abandoned' is the wordplay.
'originally' suggests taking the first letters.
'abandoned' indicates anagramming the letters.
The initial letter of 'senator' is 's'.
'a'+'roman'+'s'='aromans'
'aromans' anagrammed gives 'OARSMAN'.
(Other definitions for oarsman that I've seen before include "Member of rowing crew" , "It's bordered by Libya and France" , "One in a row" , "He rows" , "One in an eight?" .)