Keyboard note is returned to small combination (9)
I believe the answer is:
synthesis
'combination' is the definition.
(I've seen this before)
'keyboard note is returned to small' is the wordplay.
'keyboard' becomes 'synth' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'note' becomes 'e' (musical note).
'returned' is a reversal indicator.
'to small' becomes 's' (abbreviation - e.g. clothes size).
'is' back-to-front is 'si'.
'synth'+'e'+'si'+'s'='SYNTHESIS'
Can you help me to learn more?
(Other definitions for synthesis that I've seen before include "Chemical or dialectical blending process" , "getting together tricky" , "The making of compounds from simpler ones" , "Fusion" , "Process of bringing together to form a whole" .)