Large cat formerly seen round university (5)
I believe the answer is:
ounce
'large cat' is the definition.
(I've seen this before)
'formerly seen round university' is the wordplay.
'formerly' becomes 'once' (synonyms).
'seen round' indicates putting letters inside.
'university' becomes 'U' (abbreviation for university).
'once' enclosing 'u' is 'OUNCE'.
(Other definitions for ounce that I've seen before include "Approx. twenty-eight gm" , "little weight" , "one of a number in the pound" , "Small imperial weight" , "Small unit of weight" .)