Vital bit of gear arrived shortly before opening of mine (8)
I believe the answer is:
camshaft
'vital bit of gear' is the definition.
(part of a gear)
'arrived shortly before opening of mine' is the wordplay.
'arrived' becomes 'came' (come can mean to appear or arrive).
'shortly' means to remove the last letter.
'before' says to put letters next to each other.
'opening of mine' becomes 'shaft' (as in a mine shaft).
'came' with its final letter taken away is 'cam'.
'cam'+'shaft'='CAMSHAFT'
(Other definitions for camshaft that I've seen before include "Internal combustion engine valve controller" , "It operates the valves of an internal-combustion engine" , "Fat chasm (anag) - internal-combustion engine part" , "What imparts motion to engine's valves" , "Car part" .)