Anything that sounds obligatory (5)
I believe the answer is:
aught
'anything that sounds obligatory' is the definition.
'aught' can be an answer for 'anything' ('aught' means anything). I am not sure about the rest of the definition.
'anything that sounds' is the wordplay.
'anything' becomes 'ought' (dialect term for 'anything').
'that sounds' shows a homophone (sound like).
'ought' is a homophone of 'AUGHT'.
Can you help me to learn more?
(Other definitions for aught that I've seen before include "Emphatically anything" , "Nothing" , "Anything (Shakespearean)" , "Anything at all (archaic)" .)