Device to locate boy beside a river (5)
I believe the answer is:
sonar
'device' is the definition.
(sonar is a kind of device)
'boy beside a river' is the wordplay.
'boy' becomes 'son' (synonyms).
'beside' says to put letters next to each other (I've seen this in other clues).
'river' becomes 'r' (abbreviation).
'son'+'a'+'r'='SONAR'
'to locate' is the link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for sonar that I've seen before include "Echo-location system" , "Echo-sounding equipment" , "echo device" , "Device for detecting objects used in arson" , "System to locate underwater objects" .)