Top officer has realm to assemble (5,7)
I believe the answer is:
field marshal
'top officer has realm to assemble' is the definition.
I can't judge whether this defines the answer.
'top officer has realm' is the wordplay.
'top officer' becomes 'marshal' (army officer).
'has' means one lot of letters go next to another.
'realm' becomes 'field' (synonyms).
'marshal' put after 'field' is 'FIELD-MARSHAL'.
Can you help me to learn more?
(Other definitions for field marshal that I've seen before include "general? He's more important than that" , "Highest rank in British army" , "military leader" , "Highest-ranking army officer" .)