One jokes over dismissal in personnel department (6)
I believe the answer is:
hoaxer
'one jokes' is the definition.
(someone responsible for jokes or hoaxes)
'over dismissal in personnel department' is the wordplay.
'over' becomes 'o' (cricket abbreviation).
'dismissal' becomes 'axe'.
'in' is an insertion indicator.
'personnel department' becomes 'HR' (Human Resources).
'o'+'axe'='oaxe'
'oaxe' put into 'hr' is 'HOAXER'.
(Other definitions for hoaxer that I've seen before include "Someone who plays practical jokes" , "Trickster" , "Cheat such as the creator of Piltdown Man" , "He may" , "Prankster" .)