Clue: Suffering onset of 'twisties' and Simone Biles shows resilience! (15)
Answer: INDOMITABLENESS
Explanation: anagram/"Suffering" of (T and Simone Biles)* with the T taken from "onset of T-wisties" 
the gymnast Simone Biles mentioned getting the 'twisties' (feeling lost while in the air) during the Tokyo Olympics