He plays by himself (7)
I believe the answer is:
soloist
'he plays by himself' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'he' is the wordplay.
I cannot quite understand how this works, but
'he' could be 'sol' and 'sol' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for soloist that I've seen before include "musician at front of stage?" , "Lone performer" , "One who performs" , "Only one playing" , "this one's on his own" .)