Pots fired here: 50 in stock (4)
I believe the answer is:
kiln
'pots fired here 50 in stock' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.
'here 50 in stock' is the wordplay.
'here' becomes 'k' (I can't justify this - if you can you should believe this answer much more).
'50' becomes 'l' (Roman numeral).
'stock' indicates putting letters inside (some letters contain or stock others).
'l' put inside 'in' is 'iln'.
'k'+'iln'='KILN'
Can you help me to learn more?
(Other definitions for kiln that I've seen before include "Industrial furnace" , "Drying furnace" , "Industrial oven" , "Pottery furnace" , "Furnace, oven" .)