Fathom out 'cunning fool' is contradiction in terms (8)
I believe the answer is:
oxymoron
'is contradiction in terms' is the definition.
(I've seen this before)
'fathom out cunning fool' is the wordplay.
I cannot quite see how this works, but
'fool' could be 'moron' (I've seen this before) and 'moron' is located in the answer.
The remaining letters 'oxy' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for oxymoron that I've seen before include "Figure of speech, eg bitter sweet" , "Contradiction in terms" , "Paradoxical figure of speech" , "Phrase or sentence with inherent contradiction" , "'Figure of speech, it contains contradictory terms (8)'" .)