Expand space with Elgar composition (7)
I believe the answer is:
enlarge
'expand' is the definition.
(I know that expand is a more specific form of the action enlarge)
'space with elgar composition' is the wordplay.
'space' becomes 'en' (in printing, a space as wide as a letter 'n').
'with' says to put letters next to each other.
'composition' indicates an anagram (letters arranged or composed in a different order).
'elgar' is an anagram of 'large'.
'en'+'large'='ENLARGE'
(Other definitions for enlarge that I've seen before include "General (anag)" , "Expatiate" , "Amplify" , "Dilate" , "Describe in more detail" .)