More tricky keeping American housecat (6)
I believe the answer is:
mouser
'housecat' is the definition.
(I know that mouser is a type of housecat)
'more tricky keeping american' is the wordplay.
'tricky' indicates anagramming the letters.
'keeping' is an insertion indicator (some letters kept inside).
'american' becomes 'US' (abbreviation for United States).
'more' anagrammed gives 'moer'.
'moer' enclosing 'us' is 'MOUSER'.
(Other definitions for mouser that I've seen before include "Preying cat" , "One getting rid of vermin" , "moggy" , "Rodent catcher" , "Cat with certain skills" .)