Funny head of department with bun (5)
I believe the answer is:
droll
'funny' is the definition.
(droll means dryly amusing)
'head of department with bun' is the wordplay.
'head of' says to take the initial letters.
'with' means one lot of letters go next to another.
'bun' becomes 'roll' (synonyms).
The first letter of 'department' is 'd'.
'd'+'roll'='DROLL'
(Other definitions for droll that I've seen before include "Drily witty" , "Dry" , "rum" , "Whimsical - amusing" , "Clownish" .)