Those butcher boys (5)
I believe the answer is:
halal
'butcher boys' is the definition.
The definition suggests a singular noun which matches the answer.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for halal that I've seen before include "special sort of meat" , "Conforming to Muslim dietary laws" , "Meat slaughtered in accordance with Islamic law" , "Describing meat butchered according to Islamic law" , "slaughter" .)