Harmonics could be obvious ones (9)
I believe the answer is:
overtones
'harmonics' is the definition.
(overtone is a kind of harmonic)
'obvious ones' is the wordplay.
'obvious' becomes 'overt' ('overt' can be similar in meaning to 'obvious').
'overt'+'ones'='OVERTONES'
'could be' is the link.
(Other definitions for overtones that I've seen before include "Upper partials" , "added nuances" , "subtle innuendoes" , "Subtle implications" .)