Romans improved after new invaders (7)
I believe the answer is:
normans
'invaders' is the definition.
(Norman Conquest)
'romans improved after new' is the wordplay.
'improved' indicates anagramming the letters.
'after' is a charade indicator (letters next to each other).
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'romans' anagrammed gives 'ormans'.
'ormans' put after 'n' is 'NORMANS'.
(Other definitions for normans that I've seen before include "some French people" , "Ten sixty six invaders" , "They won the battle of Hastings" , "French folk" , "Invaders in 1066" .)