Get bigger when half Elgar composition follows (7)
I believe the answer is:
enlarge
'get bigger' is the definition.
(I've seen this in another clue)
'when half elgar composition follows' is the wordplay.
'half' means to take half the letters.
'composition' indicates an anagram.
'follows' is a charade indicator (letters next to each other).
'when' halved is 'en'.
'elgar' is an anagram of 'large'.
'en'+'large'='ENLARGE'
(Other definitions for enlarge that I've seen before include "Wax" , "Make greater" , "Expand" , "Widen" , "Gleaner (anag.)" .)