Choose to put detectives in river (6)
I believe the answer is:
decide
'choose to' is the definition.
(thesaurus)
'detectives in river' is the wordplay.
'detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'cid' put inside 'dee' is 'DECIDE'.
'put' is the link.
(Other definitions for decide that I've seen before include "Make up one's mind" , "Reach a conclusion" , "Determine - settle" , "Make a judgment" , "Reach a judgment" .)