### Clue: Challenge when reflective minds included hypercube (9)
### Answer: TESSERACT 
 ### Explanation: Definition: hypercube
  Reversal(*reflective*) of CARES(tends to,looks after,*minds*) in(*included*) TEST(a trial,*challenge*)
Defn:  One for the mathematicians, a *cube* with an extra dimenstion,*hyper* to the normal 3.  Can't explain more than that – it's all in the mathematician's mind.