Opening vocal variation to Elgar's First (6)
I believe the answer is:
alcove
'opening' is the definition.
(opening or recess in a building)
'vocal variation to elgar's first' is the wordplay.
'variation' is an anagram indicator.
'to elgar's first' becomes 'e' (1st letter of 'elgar').
'vocal' is an anagram of 'alcov'.
'alcov'+'e'='ALCOVE'
(Other definitions for alcove that I've seen before include "Small opening - a clove (anag)" , "Recessed part of a room" , "A small recess or niche" , "compartment" , "bay" .)