On the throne, expecting to lose head (7)
I believe the answer is:
regnant
'on the throne' is the definition.
(thesaurus)
'expecting to lose head' is the wordplay.
'expecting' becomes 'pregnant' (I've seen this before).
'to lose head' means to remove the first letter.
'pregnant' with its initial letter taken off is 'REGNANT'.
(Other definitions for regnant that I've seen before include "Current" , "Ruling" , "exercising authority" , "Reigning" , "(Queen) in her own right" .)