The more competent assembly worker (6)
I believe the answer is:
fitter
'worker' is the definition.
(someone who fits things)
I don't understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for fitter that I've seen before include "Workman" , "less likely to feel fatigue" , "Assembler; more suitable" , "Healthier" , "Tradesman being in better shape" .)