Saw inside, playing the piano (5)
I believe the answer is:
spied
'saw' is the definition.
(to spy is to see or observe)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for spied that I've seen before include "Saw" , "Was a secret agent" , "Spotted" , "Caught sight of" , "Noticed" .)