Thick poles in river (5)
I believe the answer is:
dense
'thick' is the definition.
(I know that thick can be written as dense)
'poles in river' is the wordplay.
'poles' becomes 'NS' (north and south poles).
'in' is an insertion indicator.
'river' becomes 'dee' (River Dee).
'ns' placed inside '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" .)