Either side of window that is most extensive (6)
I believe the answer is:
widest
'most extensive' is the definition.
('wide' can be similar in meaning to 'extensive')
'either side of window that is' is the wordplay.
'either side of' indicates taking the first letters (the first and last letters are the same).
'that is' becomes 'id est' ('that is' in Latin).
The first letter of 'window' is 'w'.
'w'+'idest'='WIDEST'
(Other definitions for widest that I've seen before include "most spacious" , "most off target" , "spread maximally" , "Broadest" , "showing maximal gap between sides" .)