One who often changes gear while working (5)
I believe the answer is:
model
This clue could be a double definition.
'one who often changes' is the first definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'gear while working' is the second definition.
The answer and definition can be both man-made objects as well as being singular nouns.
Maybe you can see an association between them that I can't see?
Can you help me to learn more?
(Other definitions for model that I've seen before include "(Person functioning as) example" , "Scale replica" , "Pattern" , "Perfect" , "Pose" .)