Head of quiet university isn't commonly olde-worlde (6)
I believe the answer is:
quaint
'olde-worlde' is the definition.
(I've seen this before)
'head of quiet university isn't commonly' is the wordplay.
'head of' indicates taking the first letters.
'isn't commonly' becomes 'ain't' (non-standard version of 'isn't').
The first letters of 'quiet university' is 'qu'.
'qu'+'aint'='QUAINT'
(Other definitions for quaint that I've seen before include "Attractive, in an old-fashioned way" , "Twee" , "Curious" , "Pleasantly odd" , "'Olde-worlde'" .)