Ambassador initially put label on seven-sided shape (8)
I believe the answer is:
heptagon
'seven-sided shape' is the definition.
(I've seen this before)
'ambassador initially put label on' is the wordplay.
'ambassador' becomes 'he' (His or Her Excellency).
'initially' indicates taking the first letters.
'label' becomes 'tag' (tag is a kind of label).
The first letter of 'put' is 'p'.
'he'+'p'+'tag'+'on'='HEPTAGON'
(Other definitions for heptagon that I've seen before include "Seven-sided figure" , "Seven-sided shape" , "Seven-sided object" , "A geometric figure with seven sides" .)