Surface extent of a square amphitheatre (4)
I believe the answer is:
area
'surface extent' is the definition.
(I've seen this in another clue)
'of a square amphitheatre' is the wordplay.
'of' says the answer is hidden in the clue.
'AREA' is hidden within 'square amphitheatre'.
(Other definitions for area that I've seen before include "Field of activity or surface" , "Speaking formally to (someone)" , "The measure of a surface" , "environs" , "Patch" .)