Guess shape (6)
I believe the answer is:
figure
I believe this clue is a double definition.
'guess' is the first definition.
(thesaurus)
'shape' is the second definition.
(figure is a kind of shape)
(Other definitions for figure that I've seen before include "Body shape" , "The written symbol of a number" , "Come to understand - shape" , "Consider" , "Judge to be probable, estimate" .)