In the west I had briefly been the most extensive (6)
I believe the answer is:
widest
'the most extensive' is the definition.
(I've seen this in another clue)
'in the west i had briefly' is the wordplay.
'in the' means one lot of letters goes inside another.
'i had briefly' becomes 'id' ('I'd' is a contraction of 'I had').
'west' placed around 'id' is 'WIDEST'.
'been' is the link.
I am not very happy about this link. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for widest that I've seen before include "Broadest" , "least slim" , "most unplayable for batsman?" , "spread maximally" , "showing maximal gap between sides" .)