Unhappy about hard run (4)
I believe the answer is:
dash
'run' is the definition.
(dash is a kind of run)
'unhappy about hard' is the wordplay.
'unhappy' becomes 'sad' ('sad' can be a synonym of 'unhappy').
'about' shows that the letters should be reversed in order.
'hard' becomes 'h'.
'sad' written backwards gives 'das'.
'das'+'h'='DASH'
(Other definitions for dash that I've seen before include "Pinch" , "Destroy (hopes); hyphen" , "Confound - verve" , "Splash of colour" , "Hyphen; race" .)