In travel documentation, five is one! (4)
I believe the answer is:
visa
'in travel documentation' is the definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps you can see an association between them that I don't see?
'five is one' is the wordplay.
'five' becomes 'v' (roman numeral).
'one' becomes 'a'.
'v'+'is'+'a'='VISA'
Can you help me to learn more?
(Other definitions for visa that I've seen before include "travel authority" , "Travel warrant" , "Authorisation to enter a country" , "Stamp affixed to passport as permission to enter country" , "Addition to passport" .)