Previously taking in posh cat (5)
I believe the answer is:
ounce
'cat' is the definition.
(ounce can mean a snow leopard)
'previously taking in posh' is the wordplay.
'previously' becomes 'once' (I've seen this before).
'taking in' is an insertion indicator.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'once' going around 'u' is 'OUNCE'.
(Other definitions for ounce that I've seen before include "A light weight - feline" , "Oz" , "Wildcat" , "Twelfth of a pound" , "There are quite a few in the pound" .)