Mailer novel needs better ending? (6)
I believe the answer is:
sender
'mailer' is the definition.
(I've seen this in another clue)
'novel needs better ending?' is the wordplay.
'novel' indicates an anagram (letters in a new or novel order).
'ending?' suggests the final letters (the ending of the word).
The final letter of 'better' is 'r'.
'needs' anagrammed gives 'sende'.
'sende'+'r'='SENDER'
(Other definitions for sender that I've seen before include "Message transmitter" , "Dispatcher" , "Poster" , "Transmitting device" .)