English composer Sullivan worked with him, a wag (7,8)
I believe the answer is:
vaughan williams
'english composer' is the definition.
(I've seen this before)
'sullivan worked with him a wag' is the wordplay.
'worked with' is an anagram indicator.
'him'+'a'+'wag'='himawag'
'sullivan' with its letters anagrammed with 'himawag' makes 'VAUGHAN WILLIAMS'.
(Other definitions for vaughan williams that I've seen before include "English composer" , "scorer" .)