Line written by church composer is ghastly (7)
I believe the answer is:
charnel
'ghastly' is the definition.
('charnel' can be a synonym of 'ghastly')
'line written by church composer' is the wordplay.
'line' becomes 'l'.
'written by' is a charade indicator (letters next to each other).
'church' becomes 'ch' (common abbreviation).
'composer' becomes 'arne' (I've seen this before**).
'ch'+'arne'='charne'
'l' put after 'charne' is 'CHARNEL'.
'is' is the link.
(Other definitions for charnel that I've seen before include "A - house is where bones are piled" , "Vault where bones are deposited" , "ghastly" .)