The black stuff employed by a hospital (6)
I believe the answer is:
porter
'the black stuff' is the definition.
Although both the answer and definition are singular nouns, I don't understand how they can define each other.
'employed by a hospital' is the wordplay.
'employed by' means one lot of letters goes inside another.
'a' becomes 'per'.
'hospital' becomes 'ort' (I can't justify this - if you can you should believe this answer much more).
'per' enclosing 'ort' is 'PORTER'.
Can you help me to learn more?
(Other definitions for porter that I've seen before include "Stout" , "Drink for a hotel worker" , "Hotel bag-carrier or dark beer" , "Bag carrier in hotel or railway station" , "alcoholic drink" .)