Complain that the rum's bad again (6)
I believe the answer is:
murmur
'complain' is the definition.
(murmuring is a kind of complaining)
'the rum's bad again' is the wordplay.
'bad' indicates anagramming the letters.
'again' becomes 'mur' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'rum' is an anagram of 'mur'.
'mur'+'mur'='MURMUR'
'that' acts as a link.
Can you help me to learn more?
(Other definitions for murmur that I've seen before include "Speak softly or indistinctly" , "Quiet utterance" , "Whisper; complain" , "Speak in a low voice, whisper" , "Soft voices (of discontent)" .)