A shield cut in chosen form (9)
I believe the answer is:
scutcheon
'a shield' is the definition.
(scutcheon is a kind of shield)
'cut in chosen form' is the wordplay.
'in' means one lot of letters goes inside another.
'form' is an anagram indicator (a different form of the same letters).
'chosen' anagrammed gives 'scheon'.
'cut' put into 'scheon' is 'SCUTCHEON'.
(Other definitions for scutcheon that I've seen before include "Ornamental plate" , "Shield" .)