Yuck! That is excessively short up top, generating something hard (7)
I believe the answer is:
toughie
'yuck' is the definition.
I can't tell whether this defines the answer.
'that is excessively short up top generating something hard' is the wordplay.
I cannot really see how this works, but
'hard' could be 'tough' (associated in meaning) and 'tough' is present in the answer.
'that is' could be 'ie' (short for Latin 'id est', 'that is') and 'ie' is present in the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for toughie that I've seen before include "A difficult test" , "It may prove impossible" , "DT's new difficult crossword" , "Yob" , "teaser" .)