Awful brat's cat - hard to understand (8)
I believe the answer is:
abstract
'hard to understand' is the definition.
(I've seen this before)
'awful brat's cat' is the wordplay.
'awful' is an anagram indicator.
'brats'+'cat'='bratscat'
'bratscat' is an anagram of 'ABSTRACT'.
(Other definitions for abstract that I've seen before include "a work of modern art" , "Intangible" , "Art form; remove" , "hard to understand" , "Take away - what can't be seen" .)