For tireless worker, job at last — engaged by leader for tests (5)
I believe the answer is:
robot
'for tireless worker' is the definition.
(mechanical worker)
'job at last engaged by leader for tests' is the wordplay.
'at last' suggests the final letters.
'engaged by' means one lot of letters goes inside another (I've seen this in other clues).
'leader for tests' becomes 'Root' (England test cricket captain Joe Root).
The last letter of 'job' is 'b'.
'b' going inside 'root' is 'ROBOT'.
(Other definitions for robot that I've seen before include "Soulless worker" , "Programmed machine" , "Mechanical worker" , "Mechanism that can move like a human" , "Automation" .)