Numbers are employed by him (13)
I believe the answer is:
mathematician
'numbers are employed by him' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'are employed by him' is the wordplay.
I cannot really see how this works, but
'are' could be 'a' (short for 'are', historical unit of measurement) and 'a' is found in the answer.
'by' could be 'at' and 'at' is located in the answer.
'him' could be 'mic' and 'mic' is found in the remaining letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for mathematician that I've seen before include "Euclid, say" , "Eg, Leonhard Euler" , "student engaged with combinatorics?" , "Dealer in numbers" , "Studier of organisms and their surroundings" .)