Open University’s accepting piano piece (4)
I believe the answer is:
opus
'piece' is the definition.
(I know that piece can be written as opus)
'open university's accepting piano' is the wordplay.
'open university's' becomes 'OU's'.
'accepting' is an insertion indicator (inserted letters are accepted).
'piano' becomes 'p' (musical abbreviation).
'ous' enclosing 'p' is 'OPUS'.
(Other definitions for opus that I've seen before include "Work or composition, usually musical" , "Work of 25 perhaps" , "Creative work, eg in writing, art or music" , "Composer's catalogue number" , "Showing creative skill" .)