Reckon it's a shape (6)
I believe the answer is:
figure
I believe this clue is a double definition.
'reckon' is the first definition.
(I've seen this before)
'a shape' is the second definition.
(figure is a kind of shape)
'it's' acts as a link.
(Other definitions for figure that I've seen before include "Build" , "Come to understand - shape" , "Reckon" , "Body shape" , "Diagram or digit" .)