Suffering onset of 'twisties' and Simone Biles shows resilience! (15)
I believe the answer is:
indomitableness
'resilience' is the definition.
(state of being indomitable or invincible)
'suffering onset of twisties and simone biles' is the wordplay.
'suffering' is an anagram indicator.
'onset of' suggests taking the first letters.
The first letter of 'twisties' is 't'.
't'+'and'+'simone'+'biles'='tandsimonebiles'
'tandsimonebiles' with letters rearranged gives 'INDOMITABLENESS'.
'shows' is the link.
(Another definition for indomitableness that I've seen is " determination".)