Thus composer closed (4)
I believe the answer is:
shut
'closed' is the definition.
(associated in meaning)
'thus composer' is the wordplay.
'composer' indicates an anagram.
'thus' anagrammed gives 'SHUT'.
(Other definitions for shut that I've seen before include "Slam" , "Close" , "See 19" , "Closed" , "Draw to" .)