A really reduced sector (4)
I believe the answer is:
area
'sector' is the definition.
(I've seen this in another clue)
'a really reduced' is the wordplay.
'really' becomes 'real' (synonyms).
'reduced' means to remove the last letter.
'real' with its final letter taken off is 'rea'.
'a'+'rea'='AREA'
(Other definitions for area that I've seen before include "Surface measurement" , "environs" , "Extent of a two-dimensional surface" , "Territory" , "part of town" .)