Old fool shutting in European wild cat (6)
I believe the answer is:
ocelot
'wild cat' is the definition.
(I've seen this before)
'old fool shutting in european' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'fool' becomes 'clot' (I've seen this in other clues).
'shutting in' means one lot of letters goes inside another (inserted letters are shut inside).
'european' becomes 'E' (abbreviation e.g. EU).
'clot' going around 'e' is 'celot'.
'o'+'celot'='OCELOT'
(Other definitions for ocelot that I've seen before include "Nocturnal wild cat of Central and South America" , "Striped and spotted wildcat" , "Animal in the loo etc." , "Feline of South and Central America" , "Yellow-black striped and spotted cat" .)