Attacker, one unfortunately driven out (7)
I believe the answer is:
invader
'attacker' is the definition.
(I've seen this before)
'one unfortunately driven out' is the wordplay.
'one' becomes 'a'.
'unfortunately' indicates anagramming the letters.
'out' is an insertion indicator.
'driven' with letters rearranged gives 'invder'.
'a' put into 'invder' is 'INVADER'.
(Other definitions for invader that I've seen before include "One entering by force" , "Attacker from abroad" , "far from welcome guest" , "Encroacher" , "One attacks" .)