Extreme characters engaged in a lot of sport in open space (5)
I believe the answer is:
plaza
'open space' is the definition.
(I've seen this before)
'extreme characters engaged in a lot of sport' is the wordplay.
'extreme characters' becomes 'az' (I've seen this in other clues).
'engaged in' indicates putting letters inside.
'a lot of' means to remove the last letter (most of the word but not all of it).
'sport' becomes 'play' ('play' can be a synonym of 'sport').
'play' with its last letter removed is 'pla'.
'az' inserted inside 'pla' is 'PLAZA'.
'in' acts as a link.
(Other definitions for plaza that I've seen before include "Public square, in Italy" , "Sp. square, market-place" , "Open square in Spain" , "Hotel" , "Public square for pedestrians" .)