A coral drum system is covered with heavy steel (6-4)
I believe the answer is:
armour-clad
'covered with heavy steel' is the definition.
The answer and definition are different parts of speech. However, adjectives and past participle verbs sometimes mean the same thing.
'a coral drum system' is the wordplay.
'system' indicates anagramming the letters (another version or system with the same letters).
'coral'+'drum'='coraldrum'
'coraldrum' with letters rearranged gives 'rmourclad'.
'a'+'rmourclad'='ARMOUR-CLAD'
'is' acts as a link.
Can you help me to learn more?
(Another definition for armour-clad that I've seen is " Covered by heavy metal protection".)