Playing it can involve four hands (5)
I believe the answer is:
piano
This could be a double definition.
'playing' is the first definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'it can involve four hands' is the second definition.
The definition suggests a singular noun which matches the answer.
Can you help me to learn more?
(Other definitions for piano that I've seen before include "entertain with this softly" , "Clavier" , "key holder" , "Keyboard instrument" , "Stringed instrument - to be played quietly" .)