"That's awful!" (4)
'that's awful' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for ugh that I've seen before include "Yuck (colloq.)" , "Exclamation of disgust, at 22 ac say" , "Exclamation of disgust (3)" , "Exclamation of horror" , "It's revolting" .)