Where head of state is ...? (5,4)
I believe the answer is:
morse code
'where head' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
'state is ?' is the wordplay.
I cannot quite see how this works, but
'state' could be 'or' and 'or' is found within the answer.
This explanation may well be incorrect...
'of' acts as a link.
Can you help me to learn more?
(Other definitions for morse code that I've seen before include "A way to signal" , "Dots and dashes alphabet" , "Dot-dash signalling" , "Means to communicate" , "Signalling system of dots and dashes" .)