English composer possibly lost by the hospital (5)
I believe the answer is:
holst
'english composer' is the definition.
(I've seen this before)
'possibly lost by the hospital' is the wordplay.
'possibly' is an anagram indicator.
'by' says to put letters next to each other.
'the hospital' becomes 'h' (abbreviation for hospital on maps).
'lost' anagrammed gives 'olst'.
'olst' put after 'h' is 'HOLST'.
(Other definitions for holst that I've seen before include "Scorer" , "British composer, 1874-1934" , "Gustav - (The Planets)" , "Composer of The Planets, d. 1934" , "Gustav --, English composer" .)