Killjoy started giving really unprofessional, monotonous presentation (5)
I believe the answer is:
grump
'killjoy' is the definition.
(thesaurus)
'started giving really unprofessional monotonous presentation' is the wordplay.
'started' says to take the initial letters.
The initial letters of 'giving really unprofessional monotonous presentation' is 'GRUMP'.
(Other definitions for grump that I've seen before include "Sulky type" , "Surly person" , "Beef" , "surly type" , "Crosspatch" .)