In my run, I test one entity (4)
I believe the answer is:
unit
'one entity' is the definition.
Although both the answer and definition are singular nouns, I cannot see how one could define the other.
'in my run i test' is the wordplay.
'in' says the answer is hidden in the clue.
'UNIT' is hidden within 'my run i test'.
Can you help me to learn more?
(Other definitions for unit that I've seen before include "Piece of furniture" , "Single complete domestic fixture" , "A single self-contained item" , "Entity, system; component group" , "Constituent" .)