We're told put fluid on bearing for investigation (7)
I believe the answer is:
inquest
'for investigation' is the definition.
(I know that inquest is a type of inquiry)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for inquest that I've seen before include "Coroner's inquiry into cause of death" , "Judicial enquiry" , "Inquiry into the cause of unexpected death" , "Coroner's investigation" , "Post-death investigation" .)