Empire replaced king as head of state (7)
I believe the answer is:
premier
'head of state' is the definition.
(premier is a kind of head of state)
'empire replaced king' is the wordplay.
'replaced' indicates an anagram.
'king' becomes 'r' (r is abbreviation for rex).
'empire' anagrammed gives 'premie'.
'premie'+'r'='PREMIER'
'as' is the link.
(Other definitions for premier that I've seen before include "Aberdeen or Salisbury?" , "First in importance" , "Leading" , "First - head of government" , "Chief minister" .)