One sits out with everyone on river (10)
I believe the answer is:
wallflower
'one sits' is the definition.
I can't tell whether this definition defines the answer.
'with everyone on river' is the wordplay.
'with' becomes 'w' (short for with).
'everyone' becomes 'all' (all people).
'on' means one lot of letters go next to another.
'river' becomes 'flower' (a river flows).
'w'+'all'+'flower'='WALLFLOWER'
'out' acts as a link.
Can you help me to learn more?
(Other definitions for wallflower that I've seen before include "shy sort" , "One lacking a partner - garden plant" , "Person waiting in vain to be asked to dance" , "Plant; shy person" , "one's sidelined" .)