Young island in one musical scale (5,3)
I believe the answer is:
minor key
'one musical scale' is the definition.
The definition suggests a singular noun which matches the answer.
'young island' is the wordplay.
'young' becomes 'minor' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'island' becomes 'key' (key can mean an island or reef).
'minor'+'key'='MINOR KEY'
'in' acts as a link.
Can you help me to learn more?
(Other definitions for minor key that I've seen before include "Type of scale (music)" , "Musical scale -- more inky (anag)" , "Tone" .)