Thick poles in river (5)
I believe the answer is:
dense
'thick' is the definition.
(synonyms)
'poles in river' is the wordplay.
'poles' becomes 'NS' (north and south poles).
'in' is an insertion indicator.
'river' becomes 'dee' (River Dee).
'ns' inserted into 'dee' is 'DENSE'.
(Other definitions for dense that I've seen before include "Close-packed" , "Closely crowded together" , "Tightly packed - stupid" , "Close knit" , "Closely-spaced" .)