Like diamonds leaving West Bromwich, improbably (7)
I believe the answer is:
rhombic
'like diamonds' is the definition.
(I've seen this before)
'leaving west bromwich improbably' is the wordplay.
'leaving' suggests deleting specific letters.
'west' becomes 'W' (abbreviation).
'improbably' is an anagram indicator (I've seen this in other clues).
'bromwich' with 'w' taken away is 'bromich'.
'bromich' with letters rearranged gives 'RHOMBIC'.
(Other definitions for rhombic that I've seen before include "Diamond-like" , "of lozenges" , "Quadrilateral-shaped with equal sides" , "Like diamonds" , "Shaped like a lozenge" .)