Conclude investigators drowned in river (6)
I believe the answer is:
decide
'conclude' is the definition.
(thesaurus)
'investigators drowned in river' is the wordplay.
'investigators' becomes 'cid' (I've seen this in other clues).
'drowned in' indicates putting letters inside (I've seen this in other clues).
'river' becomes 'dee' (River Dee**).
'cid' placed inside 'dee' is 'DECIDE'.
(Other definitions for decide that I've seen before include "Adjudicate" , "Make up one's mind" , "Settle conclusively" , "Resolve; give judgement" , "Reach a conclusion" .)