An A in geometry? (4)
I believe the answer is:
area
'an a in geometry?' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for area that I've seen before include "Space occupied by something" , "Extent of a 2-dimensional surface" , "Surface measurement" , "Region; extent of a surface" , "Space for a specific purpose, say picnic" .)