More unlikely to come up, looking back - that's about right (5)
I believe the answer is:
'more unlikely to come' is the definition.
I can't tell whether this definition defines the answer.
'up looking back that's about right' is the wordplay.
'up looking back' becomes 'ra' (I can't justify this - if you can you should believe this answer much more).
'that's about' becomes 're' (regarding).
'right' becomes 'r' (common abbreviation).
Can you help me to learn more?
(Other definitions for rarer that I've seen before include "even less common" , "steak may be wanted thus?" , "Comparatively sparse" , "less easy to find" , "More unusual" .)