Old Gaelic speaker possibly sheltering old cat (6)
I believe the answer is:
ocelot
'cat' is the definition.
(ocelot is a kind of cat)
'old gaelic speaker possibly sheltering old' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'gaelic speaker possibly' becomes 'celt'.
'sheltering' indicates putting letters inside.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'celt' enclosing 'o' is 'celot'.
'o'+'celot'='OCELOT'
(Other definitions for ocelot that I've seen before include "Type of American wildcat" , "Feline mammal" , "Wildcat -- loo etc (anag)" , "Small leopardlike cat" , "Fur" .)