That of a square apartment? (4)
I believe the answer is:
area
'that of a square apartment?' is the definition.
I can't judge whether this definition defines the answer.
'of a square apartment?' is the wordplay.
'of' indicates the answer is hidden within the clue.
'AREA' can be found hidden inside 'square apartment'.
Can you help me to learn more?
(Other definitions for area that I've seen before include "The measure of a surface" , "Extent of a 2-dimensional surface" , "Region; extent of a surface" , "Extent of a two-dimensional surface" , "District; locale" .)