A Greek ordeal: English with nightmare after vacation (7)
I believe the answer is:
hellene
'a greek' is the definition.
(term for an ancient Greek)
'ordeal english with nightmare after vacation' is the wordplay.
'ordeal' becomes 'hell' (both can mean a horrible experience).
'english' becomes 'E' (abbreviation).
'with' says to put letters next to each other.
'after vacation' means to remove the middle letters (the inside letters must vacate).
'nightmare' with its middle removed is 'ne'.
'hell'+'e'+'ne'='HELLENE'
(Other definitions for hellene that I've seen before include "old Greek" , "Greek girl" .)