Retired horse unhappy to race (4)
I believe the answer is:
dash
'race' is the definition.
(dash is a kind of race)
'retired horse unhappy' is the wordplay.
'retired' is a reversal indicator.
'horse' becomes 'H' (abbreviation as in horse power).
'unhappy' becomes 'sad' (associated in meaning).
'h'+'sad'='hsad'
'hsad' in reverse letter order is 'DASH'.
'to' acts as a link.
(Other definitions for dash that I've seen before include "Stroke < > to frustrate" , "Shatter - run" , "Hurry ("must ____!")" , "Kill off (hopes)" , "gratuity" .)