Ninny department leader, acting as vice principal? (10)
I believe the answer is:
dunderhead
'ninny' is the definition.
(I know that dunderhead is a type of ninny)
'vice principal?' is the wordplay.
'vice' becomes 'dunder' (I am not sure about this - if you are sure you should believe this answer much more).
'principal?' becomes 'head' (synonyms).
'dunder'+'head'='DUNDERHEAD'
'department leader acting as' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for dunderhead that I've seen before include "fool" , "dumbass?" , "Ignoramus" , "Stupid person" , "Dunce" .)