Mighty regal Elgar Variations (5)
I believe the answer is:
large
'mighty' is the definition.
(thesaurus)
'elgar variations' is the wordplay.
'variations' is an anagram indicator.
'elgar' anagrammed gives 'LARGE'.
'regal' acts as a link.
This may not be right. It may belong to another bit of the clue.
(Other definitions for large that I've seen before include "Generous" , "Massive" , "Great being regal" , "Titanic" , "Big, great" .)