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Chief and all on-the-ball engineers get a fitness certificate (5,4,2,6)

Ross

I believe the answer is:

clean bill of health

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'chief and all on-the-ball engineers get a fitness certificate' is the definition.
I don't know anything about this answer so I cannot tell whether this works.

'fitness certificate' is the wordplay.
'fitness' becomes 'clean' (I can't justify this - if you can you should believe this answer much more).
'certificate' becomes 'bill of health' (bill of health is a kind of certificate).
'clean'+'billofhealth'='CLEAN BILL OF HEALTH'

Can you help me to learn more?

(Another definition for clean bill of health that I've seen is " Statement of wellbeing".)

I've seen this clue in the Sydney Morning Herald.
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