Uniform happening to be short (4)
I believe the answer is:
'uniform' is the definition.
(I've seen this before)
'happening to be short' is the wordplay.
'happening' becomes 'event' (I've seen this in another clue).
'to be short' means to remove the last letter.
'event' with its last letter taken off is 'EVEN'.
(Other definitions for even that I've seen before include "Level as odds may be" , "Level, straight or regular" , "like six or twelve" , "Steady or smooth" , "time of day" .)