University professor's first true post (7)
I believe the answer is:
upright
'post' is the definition.
(thesaurus)
'university professor's first true' is the wordplay.
'first' indicates taking the first letters.
'true' becomes 'right' ('right' is associated in meaning with 'true').
The first letters of 'university professors' is 'up'.
'up'+'right'='UPRIGHT'
(Other definitions for upright that I've seen before include "Virtuous" , "Un-grand piano" , "Vertical post in rugby goal - strictly honourable" , "Honest like a vertical post" , "support" .)