Good to cover heavy garment (9)
I believe the answer is:
greatcoat
'heavy garment' is the definition.
'good to cover' is the wordplay.
'good' becomes 'great' (similar in meaning).
'to cover' becomes 'coat' (coating is a kind of covering).
'great'+'coat'='GREATCOAT'
(Other definitions for greatcoat that I've seen before include "Petersham, perhaps" , "One may cover officer" , "piece of clothing" , "what a soldier might wear" , "Long overgarment" .)