Expert on a large old animal (7)
I believe the answer is:
buffalo
'animal' is the definition.
(I've seen this before)
'expert on a large old' is the wordplay.
'expert' becomes 'buff' (buff can mean an expert or aficionado).
'on' means one lot of letters go next to another (in a down clue, letters appear on others).
'large' becomes 'L' (eg in clothes sizes).
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'buff'+'a'+'l'+'o'='BUFFALO'
(Other definitions for buffalo that I've seen before include "Wild animals" , "Large animal of the cattle family" , "place in New York state" , "US city" , "Bill hunted them down" .)