Big Elgar composition (5)
![Ross](/clue/static/ross-mug.min.png)
I believe the answer is:
large
'big' is the definition.
('large' can be a synonym of 'big')
'elgar composition' is the wordplay.
'composition' indicates anagramming the letters.
'elgar' with letters rearranged gives 'LARGE'.
(Other definitions for large that I've seen before include "Vast" , "Sizeable" , "Of great size" , "Ample" , "Extensive" .)