A learner in the old US university (4)
I believe the answer is:
yale
'us university' is the definition.
(I've seen this before)
'a learner in the old' is the wordplay.
'learner' becomes 'l'.
'in' indicates putting letters inside.
'the old' becomes 'ye'.
'a'+'l'='al'
'al' inserted into 'ye' is 'YALE'.
(Other definitions for yale that I've seen before include "Mass. university" , "American university or kind of lock" , "US Ivy League college" , "Thief might gain entrance after picking this" , "Educational institution" .)