Main tune hasn't complex melody at the end (6)
I believe the answer is:
shanty
'main tune' 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?
'hasn't complex melody at the end' is the wordplay.
'complex' is an anagram indicator.
'at the end' indicates one should take the final letters.
The final letter of 'melody' is 'y'.
'hasnt' anagrammed gives 'shant'.
'shant'+'y'='SHANTY'
Can you help me to learn more?
(Other definitions for shanty that I've seen before include "shed" , "Ramshackle dwelling" , "Number at sea" , "Here you get inferior accommodation" , "low abode" .)