Wicket! That is Broad, superlatively! (6)
I believe the answer is:
widest
'broad superlatively' is the definition.
I can't tell whether this defines the answer.
'wicket that is' is the wordplay.
'wicket' becomes 'w'.
'that is' becomes 'idest' ('id est' can be a synonym of 'that is').
'w'+'idest'='WIDEST'
Can you help me to learn more?
(Other definitions for widest that I've seen before include "Broadest" , "most unplayable for batsman?" , "showing maximal gap between sides" , "Most broad" , "most off target" .)