Was unhappy about the piano being cleaned up (6)
I believe the answer is:
mopped
'cleaned up' is the definition.
'mopped' can be an answer for 'cleaned' (mopping is a kind of cleaning). I am not certain of the 'up' bit.
'was unhappy about the piano' is the wordplay.
'was unhappy' becomes 'moped' (I am not sure about this - if you are sure you should believe this answer much more).
'about' indicates putting letters inside.
'the piano' becomes 'p' (musical abbreviation).
'moped' going around 'p' is 'MOPPED'.
'being' is the link.
Can you help me to learn more?
(Other definitions for mopped that I've seen before include "Cleaned or wiped with bundle on end of stick" , "Swabbed (floor)" , "Sponged (up)" .)