Mantle holder supports a dead weight (10)
I believe the answer is:
pallbearer
'a dead weight' is the definition.
Although both the answer and definition are singular nouns, I can't understand how they can define each other.
'mantle holder supports' is the wordplay.
'mantle' becomes 'pall' (synonyms).
'holder' becomes 'bearer' (bearer is a kind of holder).
'supports' says to put letters next to each other (bottom letters provide a support for the top ones).
'pall'+'bearer'='PALLBEARER'
Can you help me to learn more?
(Another definition for pallbearer that I've seen is " One carrying a coffin".)