Races occasioning unseemly haste (5)
I believe the answer is:
heats
'races' is the definition.
(I've seen this before)
'unseemly haste' is the wordplay.
'unseemly' indicates an anagram.
'haste' is an anagram of 'HEATS'.
'occasioning' acts as a link.
(Other definitions for heats that I've seen before include "Eliminating rounds in a contest" , "Eliminating rounds of a race" , "Makes hot or warm" , "Warms up for the preliminary races" , "Haste (anag.)" .)