Never go back into gymnastics. Stop (7)
I believe the answer is:
prevent
'stop' is the definition.
(stop can mean to prevent something)
'never go back into gymnastics' is the wordplay.
'go back' is a reversal indicator.
'into' means one lot of letters goes inside another.
'gymnastics' becomes 'pt' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'never' in reverse letter order is 'reven'.
'reven' inserted inside 'pt' is 'PREVENT'.
Can you help me to learn more?
(Other definitions for prevent that I've seen before include "Bar" , "Stop, hinder" , "Stop (an action)" , "Stop, avert" , "Avert, hinder" .)