Occupation university dons enter at work (6)
I believe the answer is:
tenure
'occupation' is the definition.
(I've seen this before)
'university dons enter at work' is the wordplay.
'university' becomes 'u'.
'dons' indicates putting letters inside (don means to wear or put on).
'at work' indicates an anagram.
'enter' is an anagram of 'tenre'.
'u' placed inside 'tenre' is 'TENURE'.
(Other definitions for tenure that I've seen before include "Neuter (anag)" , "Incumbency" , "Period holding office" , "Term of office" , "Dairy product" .)