Horse taking a long time over race (4)
I believe the answer is:
hare
'race' is the definition.
(hare can mean to run fast)
'horse taking a long time over' is the wordplay.
'horse' becomes 'h' (synonyms).
'taking' is a charade indicator (letters next to each other).
'a long time' becomes 'era' (a long period of time).
'over' shows that the letters should be reversed in order.
'era' back-to-front is 'are'.
'h'+'are'='HARE'
(Other definitions for hare that I've seen before include "Swift mammal" , "Run with great speed" , "Small rabbit-like mammal" , "Old saying: 'Let the .... sit" , "may be electric!" .)