Again entered AA zone (4)
I believe the answer is:
area
'zone' is the definition.
(I've seen this before)
'again entered aa' is the wordplay.
'again' becomes 're' (prefix meaning 'again').
'entered' means one lot of letters goes inside another.
're' placed within 'aa' is 'AREA'.
(Other definitions for area that I've seen before include "Extent of a 2-dimensional surface" , "Part of a building, city etc" , "land" , "District; locale" , "Extent of a two-dimensional surface" .)