Austin welcomes sextet, regularly presenting musical piece (10)
I believe the answer is:
allegretto
'musical piece' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'austin welcomes sextet regularly' is the wordplay.
'austin' becomes 'allegro' (I've seen this before).
'welcomes' means one lot of letters goes inside another (in sense of 'invites in').
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternate letters of 'sextet' are 'ett'.
'allegro' enclosing 'ett' is 'ALLEGRETTO'.
'presenting' acts as a link.
Can you help me to learn more?
(Other definitions for allegretto that I've seen before include "somewhat quick" , "Moving fairly briskly (music)" , "fairly speedily" , "at a fairly brisk rate" , "quite quickly" .)