Unhappy about hard race (4)
I believe the answer is:
dash
'race' is the definition.
(dashing is a kind of racing)
'unhappy about hard' is the wordplay.
'unhappy' becomes 'sad' (associated in meaning).
'about' says the letters should be written backwards.
'hard' becomes 'h' (abbreviation used in pencil classifications).
'sad' in reverse letter order is 'das'.
'das'+'h'='DASH'
(Other definitions for dash that I've seen before include "Smash - bother! - make a run for it" , "Hurry ("must ____!")" , "Hyphen - bother!" , "Short fast run" , "Short race run at top speed" .)