Composer composing lots after the end of March (5)
I believe the answer is:
holst
'composer' is the definition.
(Gustav Holst)
'composing lots after the end of march' is the wordplay.
'composing' indicates anagramming the letters.
'after' means one lot of letters go next to another.
'the end of' says to take the final letters.
The last letter of 'march' is 'h'.
'lots' anagrammed gives 'olst'.
'olst' put after 'h' is 'HOLST'.
(Other definitions for holst that I've seen before include "He scored" , "Planets Suite composer" , "Gustav - (The Planets)" , "Composer of The Planets, d. 1934" , "Scorer" .)