Chief and all on-the-ball engineers get a fitness certificate (5,4,2,6)
I believe the answer is:
clean bill of health
'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".)