Once-in-a-lifetime right date shows up (4)
I believe the answer is:
rare
'once-in-a-lifetime' is the definition.
(I've seen this in another clue)
'right date shows up' is the wordplay.
'right' becomes 'r' (common abbreviation).
'date' becomes 'era' (I've seen this before).
'shows up' says the letters should be written in reverse.
'era' back-to-front is 'are'.
'r'+'are'='RARE'
(Other definitions for rare that I've seen before include "like gold dust" , "Not easily come by like undercooked meat" , "Meat that's lightly cooked" , "Very scarce like valuable antique" , "Very scarce, like undercooked steak" .)