Old-fashioned quadrangle (6)
I believe the answer is:
square
This clue is a double definition.
'old-fashioned' is the first definition.
(I've seen this before)
'quadrangle' is the second definition.
(I know that square is a type of rhombus)
(Other definitions for square that I've seen before include "Right-angled; plaza" , "Level - shape - piazza - stuffy" , "Once bore" , "See 19" , "bashed by soldiers in training?" .)