Thick poles set in the river (5)
I believe the answer is:
dense
'thick' is the definition.
(synonyms)
'poles set in the river' is the wordplay.
'poles' becomes 'ns' (North and South poles).
'set in' is an insertion indicator.
'the river' becomes 'dee' (River Dee).
'ns' put inside 'dee' is 'DENSE'.
(Other definitions for dense that I've seen before include "Tightly packed - stupid" , "Heavy" , "Thick as undergrowth may be" , "Close-knit" , "Slow-witted" .)