Thus composer closed (4)
I believe the answer is:
shut
'closed' is the definition.
(I know that closed can be written as shut)
'thus composer' is the wordplay.
'composer' indicates anagramming the letters.
'thus' is an anagram of 'SHUT'.
(Other definitions for shut that I've seen before include "Slam" , "Close" , "See 19" , "Closed" , "Draw to" .)