Left old university? Not entirely true (5)
I believe the answer is:
loyal
'true' is the definition.
(I've seen this before)
'left old university? not entirely' is the wordplay.
'left old' becomes 'lo' (this might be a standard abbreviation which I don't know about).
'university?' becomes 'Yale' (Yale is an example).
'not entirely' means to remove the last letter.
'yale' with its final letter taken away is 'yal'.
'lo'+'yal'='LOYAL'
(Other definitions for loyal that I've seen before include "Sort of toast" , "Solid" , "Staunch" , "True-hearted" , "Steadfast and true" .)