The cat gets us in more trouble (6)
I believe the answer is:
mouser
'the cat' is the definition.
(mouser is a kind of cat)
'us in more trouble' is the wordplay.
'in' means one lot of letters goes inside another.
'trouble' indicates anagramming the letters.
'more' anagrammed gives 'moer'.
'us' going inside 'moer' is 'MOUSER'.
'gets' acts as a link.
(Other definitions for mouser that I've seen before include "Predatory cat" , "moggy" , "Cat as hunter of small rodents" , "Preying cat" , "Rodent catcher" .)