More unlikely to come up, looking back - that's about right (5)
I believe the answer is:
rarer
'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).
'ra'+'re'+'r'='RARER'
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" .)