Hard nut, so sure to get a beating (9)
I believe the answer is:
strenuous
'hard' is the definition.
(I know that hard can be written as strenuous)
'nut so sure to get a beating' is the wordplay.
'to get a beating' indicates an anagram.
'nut'+'so'+'sure'='nutsosure'
'nutsosure' anagrammed gives 'STRENUOUS'.
(Other definitions for strenuous that I've seen before include "Very active" , "Needing much effort" , "Vigorous" , "Taxing" , "with difficulty" .)