Cat wants mouse's heart for stuffing as soon as (5)
I believe the answer is:
ounce
'cat' is the definition.
(ounce can mean a snow leopard)
'mouse's heart for stuffing as soon as' is the wordplay.
'heart' indicates the central letters.
'for stuffing' means one lot of letters goes inside another.
'as soon as' becomes 'once' (once can mean immediately after).
The middle of 'mouse' is 'u'.
'u' put within 'once' is 'OUNCE'.
'wants' is the link.
(Other definitions for ounce that I've seen before include "Unit equal to about 28 grams" , "Imperial unit of weight" , "Atom" , "Wildcat" , "Enough to tip the scales" .)