Old fool grabbing tail of Siamese cat (6)
I believe the answer is:
ocelot
'cat' is the definition.
(ocelot is a kind of cat)
'old fool grabbing 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).
'grabbing' is an insertion indicator.
'tail of' suggests the final letters.
The last letter of 'siamese' is 'e'.
'clot' going around 'e' is 'celot'.
'o'+'celot'='OCELOT'
(Other definitions for ocelot that I've seen before include "American cat, like a small leopard" , "Small leopardlike cat" , "Wild cat, native to Central and South America - loo etc (anag)" , "Yellow-black striped and spotted cat" , "Wild creature" .)