Wild cat on a single occasion grabbing coypu's tail (5)
I believe the answer is:
ounce
'wild cat' is the definition.
(I've seen this before)
'on a single occasion grabbing coypu's tail' is the wordplay.
'on a single occasion' becomes 'once' (I've seen this before).
'grabbing' indicates putting letters inside.
'tail' suggests the final letters.
The last letter of 'coypu' is 'u'.
'once' enclosing 'u' is 'OUNCE'.
(Other definitions for ounce that I've seen before include "Small amount (of old pound)" , "Imperial unit of weight" , "28.3495 grammes" , "Wild cat - small weight" , "one of a number in the pound" .)