Base x height, for a parallelogram (4)
I believe the answer is:
area
'base x height for a parallelogram' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for area that I've seen before include "Length multiplied by breadth" , "District; locale" , "Region, zone" , "Patch" , "Speaking formally to (someone)" .)