Such pigeons are humanely shot (4)
I believe the answer is:
clay
'humanely shot' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'such pigeons are' is the wordplay.
I cannot quite see how this works, but
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is located in the answer.
The remaining letters 'cly' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for clay that I've seen before include "Sticky earth; human substance (poet.)" , "slip" , "Material used by potter" , "Earthenware" , "Pigeon target" .)