Determined detectives in river died (7)
I believe the answer is:
decided
'determined' is the definition.
(synonyms)
'detectives in river died' is the wordplay.
'detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'in' indicates putting letters inside.
'river' becomes 'dee' (River Dee).
'died' becomes 'd' (abbreviation next to the year of someone's death).
'cid' put inside 'dee' is 'decide'.
'decide'+'d'='DECIDED'
(Other definitions for decided that I've seen before include "Definite; agreed" , "Indisputable" , "Made up one's mind" , "Resolute; settled" , "Unquestionable" .)