Old fool grabs tail of Siamese cat (6)
I believe the answer is:
ocelot
'cat' is the definition.
(ocelot is a kind of cat)
'old fool grabs tail of siamese' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'fool' becomes 'clot' (I've seen this before).
'grabs' indicates putting letters inside.
'tail of' suggests the final letters.
The final letter of 'siamese' is 'e'.
'clot' placed around 'e' is 'celot'.
'o'+'celot'='OCELOT'
(Other definitions for ocelot that I've seen before include "Feline mammal" , "Striped and spotted wildcat" , "Animal in the loo etc." , "Wildcat -- loo etc (anag)" , "Type of American wildcat" .)