Choose to drop policemen in river (6)
I believe the answer is:
decide
'choose to' is the definition.
(thesaurus)
'policemen in river' is the wordplay.
'policemen' becomes 'CID' (abbreviation for Criminal Investigation Department).
'in' means one lot of letters goes inside another.
'river' becomes 'dee' (River Dee).
'cid' put within 'dee' is 'DECIDE'.
'drop' acts as a 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 "Reach a judgment" , "Judge" , "Conclude" , "Choose" , "Resolve or give a judgment" .)