Notice short ambassador's hat (6)
I believe the answer is:
cloche
'hat' is the definition.
(I've seen this before)
'notice short ambassador's' is the wordplay.
'notice' becomes 'clock' (I've seen this before).
'short' means to remove the last letter.
'ambassador' becomes 'he' (His or Her Excellency).
'clock' with its last letter taken off is 'cloc'.
'cloc'+'he'='CLOCHE'
(Other definitions for cloche that I've seen before include "Cover put over young plants to protect them from cold" , "Bell-shaped hat" , "Plant cover; hat type" , "Transparent cover for forcing plants" , "It's old hat!" .)