“Remaining coherent to the last” is too difficult a clue (9)
I believe the answer is:
insoluble
'remaining coherent' is the definition.
'insoluble' can be an answer for 'coherent' (thesaurus). I'm not sure about the 'remaining' bit.
'the last is too difficult a clue' is the wordplay.
I cannot quite see how this works, but
'too' could be 'so' ('so' can be a synonym of 'too') and 'so' is present in the answer.
This explanation may well be incorrect...
'to' acts as a link.
Can you help me to learn more?
(Other definitions for insoluble that I've seen before include "I hope this isn't" , "Of problem or puzzle, can't be worked out" , "Impossible to solve" , "Utterly baffling" , "Incapable of being dissolved" .)