First empire smashed by king (7)
I believe the answer is:
premier
'first' is the definition.
(similar in meaning)
'empire smashed by king' is the wordplay.
'smashed' is an anagram indicator.
'by' means one lot of letters go next to another.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'empire' anagrammed gives 'premie'.
'premie'+'r'='PREMIER'
(Other definitions for premier that I've seen before include "First in importance" , "First in rank or degree" , "Foremost" , "Which is having precedence" , "Earliest" .)