Solitary person left one beside river (5)
I believe the answer is:
loner
'solitary person' is the definition.
(I've seen this before)
'left one beside river' is the wordplay.
'left' becomes 'l' (common abbreviation).
'beside' says to put letters next to each other (I've seen this in other clues).
'river' becomes 'r' (abbreviation).
'l'+'one'+'r'='LONER'
(Other definitions for loner that I've seen before include "Solitary type" , "Person shunning company" , "He's not a joiner" , "Recluse" , "Solitary individual, prefers own company" .)