Maybe flageolet's one joining piano in exercises (4)
I believe the answer is:
pipe
'maybe flageolet's' is the definition.
I can't tell whether this defines the answer.
'one joining piano in exercises' is the wordplay.
'one' becomes 'i' (Roman numeral).
'joining' indicates putting letters inside (inserted letters join the word).
'piano' becomes 'p' (musical abbreviation).
'in' means one lot of letters goes inside another.
'exercises' becomes 'pe' (Physical Education).
'p' put within 'pe' is 'ppe'.
'i' inserted inside 'ppe' is 'PIPE'.
Can you help me to learn more?
(Other definitions for pipe that I've seen before include "11th" , "Possibly a churchwarden" , "Utter shrilly; wine cask" , "Tropical fruit" , "Tube; wind instrument" .)