An improved worker? (6)
I believe the answer is:
fitter
'worker?' is the definition.
(someone who fits things)
I don't understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for fitter that I've seen before include "Healthier" , "less likely to feel fatigue" , "Mechanic" , "Workman" , "better equipped" .)