Sunday goes by, cryptic is ultimately unsolvable (8,5)
I believe the answer is:
anybodys guess
'sunday goes by cryptic is ultimately unsolvable' is the definition.
I can't judge whether this defines the answer.
'sunday goes by cryptic is ultimately' is the wordplay.
'cryptic' indicates an anagram (a cryptic or obscure order of the letters).
'ultimately' indicates one should take the final letters.
The final letter of 'is' is 's'.
'sunday'+'goes'+'by'='sundaygoesby'
'sundaygoesby' with letters rearranged gives 'anybodysgues'.
'anybodysgues'+'s'='ANYBODYS GUESS'
Can you help me to learn more?
(Other definitions for anybodys guess that I've seen before include "Unknown" , "Very difficult to determine" , "hard to say" , "Impossible to know" , "A matter for general conjecture" .)