Silence head of administration, slightly crazy (4)
I believe the answer is:
gaga
'slightly crazy' is the definition.
(I've seen this before)
'silence head of administration' is the wordplay.
'silence' becomes 'gag' (gagging is a kind of silencing).
'head of' suggests taking the first letters.
The initial letter of 'administration' is 'a'.
'gag'+'a'='GAGA'
(Other definitions for gaga that I've seen before include "fatuous?" , "Slang for crazy - apply a gag" , "Very fond of" , "Foolishly doting" , "Lady < > in a senile" .)