A country invaded by former queen, nominally great ruler (9)
I believe the answer is:
alexander
'nominally great ruler' is the definition.
(Alexander the Great)
'a country invaded by former queen' is the wordplay.
'country' becomes 'land' (synonyms).
'invaded by' is an insertion indicator.
'former' becomes 'ex'.
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'land' going around 'ex' is 'lexand'.
'a'+'lexand'+'er'='ALEXANDER'
(Other definitions for alexander that I've seen before include "Great king of ancient Macedonia" , "A Great king of old Macedon" , "Healing process based on improving posture" , "Macedonian conqueror of Greece" , "Great king; --- Pope, poet" .)