Top of wall that is hardest to jump over? (6)
I believe the answer is:
widest
'hardest to jump over?' is the definition.
'top of wall that is' is the wordplay.
'top of' indicates taking the first letters.
'that is' becomes 'id est' ('that is' in Latin).
The first letter of 'wall' is 'w'.
'w'+'idest'='WIDEST'
(Other definitions for widest that I've seen before include "Most broad" , "most off target" , "least slim" , "spread maximally" , "most spacious" .)