His work's a grind (6)
I believe the answer is:
miller
'his work's a' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'grind' is the wordplay.
I cannot really understand how this works, but
'grind' could be 'mill' (milling is a kind of grinding) and 'mill' is present in the answer.
The remaining letters 'er' 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 miller that I've seen before include "Grain grinder" , "He makes flour" , "Cereal worker" , "Grinder of flour" , "Producer of flour" .)