Flustered professor's beginning to interrupt university class (5)
I believe the answer is:
upset
'flustered' is the definition.
(thesaurus)
'professor's beginning to interrupt university class' is the wordplay.
'beginning to' says to take the initial letters.
'interrupt' is an insertion indicator.
'university' becomes 'u'.
'class' becomes 'set' (I've seen this before**).
The initial letter of 'professors' is 'p'.
'u'+'set'='uset'
'p' going within 'uset' is 'UPSET'.
(Other definitions for upset that I've seen before include "Disturbed or capsized" , "Overturn, tip over" , "Aggrieved" , "Disturb or knock over" , "Disturb, physically or emotionally" .)