Incredible mind a genius endlessly exercised (10)
I believe the answer is:
unimagined
'incredible' is the definition.
(similar in meaning)
'mind a genius endlessly exercised' is the wordplay.
'endlessly' means to remove the last letter.
'exercised' is an anagram indicator.
'mind'+'a'+'genius'='mindagenius'
'mindagenius' with its final letter taken off is 'mindageniu'.
'mindageniu' anagrammed gives 'UNIMAGINED'.
(Other definitions for unimagined that I've seen before include "Not thought of" , "Not foreseen" .)