Police in river to reach conclusion (6)
I believe the answer is:
decide
'to reach conclusion' is the definition.
I can't tell whether this defines the answer.
'police in river' is the wordplay.
'police' becomes 'CID' (abbreviation for Criminal Investigation Department).
'in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'cid' going within 'dee' is 'DECIDE'.
Can you help me to learn more?
(Other definitions for decide that I've seen before include "Judge" , "Choose" , "Adjudicate" , "Make a judgment" , "Reach a conclusion" .)