Very rarely used in the past while arguing with Korean president (4,2,1,4,4)
I believe the answer is:
once in a blue moon
'very rarely' is the definition.
(I've seen this in another clue)
'in the past while arguing with korean president' is the wordplay.
I cannot quite see how this works, but
'in' is within the answer.
'past' could be 'once' (similar in meaning) and 'once' is found within the answer.
This explanation may well be incorrect...
'used' is the link.
Can you help me to learn more?
(Other definitions for once in a blue moon that I've seen before include "very rarely" , "on very rare occasions" , "very occasionally" , "with extremely low frequency'" , "Hardly ever (4,2,1,4,4)" .)