Composer of The Planets turning out large Mass (4)
I believe the answer is:
host
'mass' is the definition.
(thesaurus)
'composer of the planets turning out large' is the wordplay.
'composer of the planets' becomes 'holst' (Gustav Holst composed The Planets).
'turning out' is a deletion indicator.
'large' becomes 'L' (eg in clothes sizes).
'holst' with 'l' removed is 'HOST'.
(Other definitions for host that I've seen before include "lot of people" , "Entertainer of guests" , "Multitude - emcee" , "TV presenter; large number" , "Army - innkeeper" .)