The cat gets us more upset (6)
I believe the answer is:
mouser
'the cat' is the definition.
(mouser is a kind of cat)
'us more upset' is the wordplay.
'upset' indicates anagramming the letters.
'us'+'more'='usmore'
'usmore' with letters rearranged gives 'MOUSER'.
'gets' acts as a link.
(Other definitions for mouser that I've seen before include "Rodent catcher" , "Cat with certain skills" , "Predatory cat" , "Cat that's good at catching small rodents" , "One getting rid of vermin" .)