Prime Minister would vote for emergency appeal (6)
I believe the answer is:
mayday
'emergency appeal' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'prime minister would vote for' is the wordplay.
'prime minister' becomes 'may' (I've seen this in other clues).
'would' becomes 'd' (as in 'we'd' - 'we would').
'vote for' becomes 'ay' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'may'+'d'+'ay'='MAYDAY'
Can you help me to learn more?
(Other definitions for mayday that I've seen before include "Bank holiday - distress signal" , "International radio distress signal" , "Request for help" , "I'm in trouble" , "When the workers celebrate" .)