Choose to throw investigators into the river (6)
I believe the answer is:
decide
'choose to' is the definition.
(thesaurus)
'investigators into the river' is the wordplay.
'investigators' becomes 'CID' (Criminal Investigation Department).
'into' is an insertion indicator.
'the river' becomes 'dee' (River Dee).
'cid' inserted within 'dee' is 'DECIDE'.
'throw' is the link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for decide that I've seen before include "Come to a resolution" , "Settle conclusively" , "Reach a conclusion" , "Conclude" , "Reach a judgment" .)