The Planets composer (5)
I believe the answer is:
holst
'the planets composer' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for holst that I've seen before include "Gustav - (The Planets)" , "Gustav --, English composer" , "British composer, 1874-1934" , "Composer of The Planets, d. 1934" , "Planets Suite composer" .)