Anybody who isn't off key (3)
I believe the answer is:
one
'anybody' is the definition.
(I've seen this in another clue)
'isn't off key' is the wordplay.
'isn't off' becomes 'on' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'key' becomes 'e' (a musical key).
'on'+'e'='ONE'
'who' is the link.
Can you help me to learn more?
(Other definitions for one that I've seen before include "Half of two" , "1" , "Hour after midnight" , "Leading cardinal" , "both halves" .)