Composer who could be regal (5)
I believe the answer is:
elgar
'composer' is the definition.
(Edward Elgar)
'could be regal' is the wordplay.
'could be' indicates anagramming the letters.
'regal' is an anagram of 'ELGAR'.
'who' is the link.
(Other definitions for elgar that I've seen before include "13 composer, 1857-1934" , "Glare critically at the British composer" , "Nimrod composer" , "Sir Edward -, English composer" , "Score-person" .)