Diverting sleet – that’s hard (5)
I believe the answer is:
steel
'hard' is the definition.
'diverting sleet' is the wordplay.
'diverting' indicates an anagram (letters diverted elsewhere).
'sleet' is an anagram of 'STEEL'.
'that's' is the link.
(Other definitions for steel that I've seen before include "Iron/carbon alloy" , "Strength, determination" , "Hard, strong, grey alloy" , "Mixture of carbon and iron" , "The staying power" .)