Resolve of detectives to enter river (6)
I believe the answer is:
decide
'resolve' is the definition.
(I know that resolve can be written as decide)
'detectives to enter river' is the wordplay.
'detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'to enter' means one lot of letters goes inside another.
'river' becomes 'dee' (River Dee).
'cid' going inside 'dee' is 'DECIDE'.
'of' acts as a link.
(Other definitions for decide that I've seen before include "Reach a conclusion" , "Choose" , "Make a judgment" , "Judge" , "Resolve, settle (issue)" .)