Cat upset by bat (5)
I believe the answer is:
tabby
'cat' is the definition.
(I know that tabby is a type of cat)
'upset by bat' is the wordplay.
'upset' indicates an anagram.
'by'+'bat'='bybat'
'bybat' with letters rearranged gives 'TABBY'.
(Other definitions for tabby that I've seen before include "Brindled cat" , "Cat with a mottled coat" , "Cat with striped markings" , "Grey-brown cat" , "Mottled and striped grey or brownish cat" .)