River for you to sit beside somewhere in Sussex (3)
I believe the answer is:
rye
'somewhere in sussex' is the definition.
(Rye is a town in East Sussex)
'river for you' is the wordplay.
'river' becomes 'r' (abbreviation for river).
'for' is a charade indicator (letters next to each other).
'you' becomes 'ye' (historical version of 'you').
'r'+'ye'='RYE'
'to sit beside' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
(Other definitions for rye that I've seen before include "21 [GRASS]" , "New York bread" , "place in East Sussex" , "Hardy cereal crop" , "Gypsy gentleman" .)