One old lady with pizzazz is astonishing (7)
I believe the answer is:
amazing
'astonishing' is the definition.
('amazing' can be a synonym of 'astonishing')
'one old lady with pizzazz' is the wordplay.
'one' becomes 'a'.
'old lady' becomes 'ma' (**).
'with' is a charade indicator (letters next to each other).
'pizzazz' becomes 'zing' ('zing' can be a synonym of 'pizzazz').
'a'+'ma'+'zing'='AMAZING'
'is' is the link.
(Other definitions for amazing that I've seen before include "Excellent; surprising" , "Very impressive" , "Causing great surprise" , "Stunning, surprising" , "Causing wonder" .)