Murder composer that’s challenging, cold and dangerous (7)
I believe the answer is:
iceberg
'challenging cold and dangerous' is the definition.
I can't tell whether this defines the answer.
'murder composer' is the wordplay.
'murder' becomes 'ice' (slang term).
'composer' becomes 'berg' (20th Century Austrian composer).
'ice'+'berg'='ICEBERG'
'that's' is the link.
Can you help me to learn more?
(Other definitions for iceberg that I've seen before include "Floating piece from glacier" , "A floating ice mountain" , "Titanic sinker - lettuce" , "Wrecker of Titanic" , "Hazard in Arctic sea" .)