Metal worker almost entirely engaged by corporation (8)
I believe the answer is:
tantalum
'metal' is the definition.
(tantalum is a kind of metal)
'worker almost entirely engaged by corporation' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'almost' means to remove the last letter (most of the word but not all of it).
'entirely' becomes 'all' (I've seen this before).
'engaged by' is an insertion indicator (I've seen this in other clues).
'corporation' becomes 'tum' (I've seen this before).
'all' with its final letter taken off is 'al'.
'ant'+'al'='antal'
'antal' going into 'tum' is 'TANTALUM'.
(Other definitions for tantalum that I've seen before include "Metallic element, symbol Ta" , "Acid-resistant metal" .)