Such a speech is a tonic (7)
I believe the answer is:
keynote
This clue is a double definition.
'such a speech' is the first definition.
(keynote speech is a kind of speech)
'tonic' is the second definition.
('keynote' can be a synonym of 'tonic')
'is a' is the link.
(Other definitions for keynote that I've seen before include "of fundamental importance" , "Prevailing tone or idea of a speech" , "Central theme" , "Speech that sets the tone of a conference" , "Main theme - tonic" .)