Younger relative is given a hand system for underwater detection (5)
I believe the answer is:
'underwater detection' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'younger relative is given a hand system' is the wordplay.
'younger relative' becomes 'son'.
'is given' is a charade indicator (letters next to each other).
'hand' becomes 'R' (abbreviation for right, as in the right-hand side).
'system' is an anagram indicator (another version or system with the same letters).
'a' put after 'r' is 'ra'.
'ra' is an anagram of 'ar'.
'for' is the link.
Can you help me to learn more?
(Other definitions for sonar that I've seen before include "Echo sounder - arson (anag)" , "Undersea sensing system" , "Detection system at sea" , "Underwater detection system" , "Underwater location equipment" .)