View, finally, that is most extensive (6)
I believe the answer is:
widest
'most extensive' is the definition.
('wide' can be similar in meaning to 'extensive')
'view finally that is' is the wordplay.
'finally' indicates one should take the final letters.
'that is' becomes 'id est' ('that is' in Latin).
The final letter of 'view' is 'w'.
'w'+'idest'='WIDEST'
(Other definitions for widest that I've seen before include "most off target" , "showing maximal gap between sides" , "most spacious" , "least slim" , "most unplayable for batsman?" .)