My herb in a floral arrangement (7)
I believe the answer is:
corsage
'a floral arrangement' is the definition.
(arrangement of flowers worn on clothing)
'my herb' is the wordplay.
'my' becomes 'cor' (both are expressions of surprise).
'herb' becomes 'sage' (sage is a kind of herb).
'cor'+'sage'='CORSAGE'
'in' acts as a link.
(Other definitions for corsage that I've seen before include "Flower worn on bodice" , "Flowers pinned to dress" , "Part of dress" , "Sort of bouquet" , "Nosegay, posy" .)