Perhaps Mass composed in Antarctica almost killed composer (7)
I believe the answer is:
iceberg
'perhaps mass composed in antarctica' is the definition.
(a mass of ice)
'almost killed composer' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'killed' becomes 'iced' (ice can informally mean to assassinate).
'composer' becomes 'berg' (20th Century Austrian composer).
'iced' with its last letter taken away is 'ice'.
'ice'+'berg'='ICEBERG'
(Other definitions for iceberg that I've seen before include "Floating sea hazard" , "RMS Titanic's nemesis" , "main danger" , "Type of 16 Down" , "Frozen floater" .)