Small amount at one time limiting university (5)
I believe the answer is:
ounce
'small amount' is the definition.
(an ounce is a small amount)
'at one time limiting university' is the wordplay.
'at one time' becomes 'once' (both can mean 'in the past').
'limiting' 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 "Lightweight" , "Snow-leopard" , "Small amount (of old pound)" , "Imperial unit of weight" , "Wild cat - small weight" .)