### Clue: Cryptic clue I finally solved (being brilliant at maths) (6)
### Answer: EUCLID 
 ### Explanation: Definition: being brilliant at maths
A charade of EUCL, an anagram ('cryptic') of 'clue' plus 'I' plus D ('finally solve**D**'). In the definition, I take 'being' as a noun, a person.