Wally in the role of head of state (3)
I believe the answer is:
ass
'wally' is the definition.
(I've seen this before)
'in the role of head of state' is the wordplay.
'in' means one lot of letters goes inside another.
'the role of' becomes 'as' (I've seen this in other clues).
'head of' says to take the initial letters.
The initial letter of 'state' is 's'.
'as' placed around 's' is 'ASS'.
(Other definitions for ass that I've seen before include "Braying animal" , "Idiot; animal" , "transporter" , "sap" , "Pompous person" .)