Individual on key (3)
I believe the answer is:
one
'individual' is the definition.
(an individual is one person)
'on key' is the wordplay.
'key' becomes 'e' (musical key).
'on'+'e'='ONE'
(Other definitions for one that I've seen before include "Hour after midnight" , "SECOND PART OF 16" , "Single and undivided" , "Single person or thing" , "doesn't count for much" .)