It's rum, is rum! (6)
I believe the answer is:
truism
'it's rum is rum' is the definition.
I can't tell whether this defines the answer.
'it's rum is rum' is the wordplay.
'is rum' indicates an anagram (rum can mean strange or unusual).
'its'+'rum'='itsrum'
'itsrum' with letters rearranged gives 'TRUISM'.
Can you help me to learn more?
(Other definitions for truism that I've seen before include "An abvious truth" , "Obviously correct and verified statement" , "Something obvious" , "Bromide" , "Saying that's obviously accurate" .)