Happening once in a blue moon, red! (4)
I believe the answer is:
rare
'red' is the definition.
(thesaurus)
I can't explain the rest of the clue.
Can you help me to learn more?
(Other definitions for rare that I've seen before include "Infrequent; remarkable" , "Not found often" , "of low density" , "Not well done!" , "Unusual(ly fine)" .)