Advance and squat by river (5)
I believe the answer is:
posit
'advance' is the definition.
(I've seen this before)
'squat by river' is the wordplay.
'squat' becomes 'sit' (I've seen this in other clues).
'by' is a charade indicator (letters next to each other).
'river' becomes 'po' (Po is an example).
'sit' after 'po' is 'POSIT'.
'and' acts as a link.
(Other definitions for posit that I've seen before include "Suggest" , "Put forward (as fact)" , "forward-looking statement" , "given proposition" , "state" .)