One church composer, a big cold lump! (7)
I believe the answer is:
iceberg
'a big cold lump' is the definition.
(lump of ice)
'one church composer' is the wordplay.
'one' becomes 'i' (Roman numeral).
'church' becomes 'CE' (abbreviation for Church of England).
'composer' becomes 'Berg' (20th Century Austrian composer).
'i'+'ce'+'berg'='ICEBERG'
(Other definitions for iceberg that I've seen before include "Hazard of polar seas" , "threat to ship" , "last scene of Titanic" , "main danger" , "One sank the Titanic" .)