Toughest part — that's horrible! (3)
I believe the answer is:
ugh
'that's horrible' is the definition.
(expression of disgust)
'toughest part' is the wordplay.
'part' indicates a hidden word.
'UGH' can be found hidden inside 'toughest'.
(Other definitions for ugh that I've seen before include "Exclamation of horror" , "Yuck (colloq.)" , "Expression of disgust" , "Exclamation of disgust (3)" , "expression of revulsion" .)