Fixture, not ending all square (4)
I believe the answer is:
even
'all square' is the definition.
(I've seen this before)
'fixture not ending' is the wordplay.
'fixture' becomes 'event' (I've seen this before).
'not ending' means to remove the last letter.
'event' with its final letter taken away is 'EVEN'.
(Other definitions for even that I've seen before include "Level, straight or regular" , "of consistent quality" , "Deep crisp snow seen on Boxing Day also this" , "Flush" , "Flattened" .)