Too difficult, this clue will be (8)
I believe the answer is:
unsolved
'difficult this clue will be' is the definition.
The definition and answer are not the same part of speech.
'too' is the wordplay.
I cannot really see how this works, but
'too' could be 'so' (synonyms) and 'so' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for unsolved that I've seen before include "Still a puzzle like old Venus" , "Not yet explained" , "Having no answer" , "Not worked out" .)