Funny head of department with register (5)
I believe the answer is:
droll
'funny' is the definition.
(droll means dryly amusing)
'head of department with register' is the wordplay.
'head of' indicates taking the first letters.
'with' says to put letters next to each other.
'register' becomes 'roll' (list of names).
The initial letter of 'department' is 'd'.
'd'+'roll'='DROLL'
(Other definitions for droll that I've seen before include "Large amount (of matter)" , "Humorous or whimsical" , "Whimsically comical" , "Funny" , "Drily witty" .)