Harmonic apparent on single (8)
I believe the answer is:
overtone
'harmonic' is the definition.
(overtone is a kind of harmonic)
'apparent on single' is the wordplay.
'apparent' becomes 'overt' (I've seen this before).
'on' says to put letters next to each other.
'single' becomes 'one' (I've seen this before).
'overt'+'one'='OVERTONE'
(Other definitions for overtone that I've seen before include "Subtle extra meaning" , "Hint" , "Subtle quality" , "hidden meaning" , "Association" .)