Sang atrociously at the piano, sheer agony! (5)
I believe the answer is:
pangs
'agony' is the definition.
(thesaurus)
'sang atrociously at the piano sheer' is the wordplay.
I cannot really understand how this works, but
an anagram of 'sang' is 'angs' which is located in the answer.
'piano' could be 'p' (musical abbreviation) and 'p' is located in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Another definition for pangs that I've seen is " Sudden pains".)