Cat more unlikely to impress us (6)
I believe the answer is:
mouser
'cat' is the definition.
(mouser is a kind of cat)
'more unlikely to impress us' is the wordplay.
'unlikely' indicates anagramming the letters.
'to impress' means one lot of letters goes inside another (impress can mean enlist or recruit).
'more' with letters rearranged gives 'moer'.
'moer' going around 'us' is 'MOUSER'.
(Other definitions for mouser that I've seen before include "Useful mog" , "Cat proficient at catching small rodents" , "Rodent catcher" , "Preying cat" , "Rodent-catching cat" .)