Empire collapsed: king summons PM (7)
I believe the answer is:
premier
'summons pm' is the definition.
'premier' can be an answer for 'pm' (synonyms). I'm not sure about the 'summons' bit.
'empire collapsed king' is the wordplay.
'collapsed' is an anagram indicator.
'king' becomes 'R' (abbreviation of Latin rex).
'empire' with letters rearranged gives 'premie'.
'premie'+'r'='PREMIER'
Can you help me to learn more?
(Other definitions for premier that I've seen before include "Earliest" , "Which is having precedence" , "First in rank or degree" , "First in importance" , "Leading" .)