Check others taking exercise inside (9)
I believe the answer is:
restraint
'check' is the definition.
(I've seen this before)
'others taking exercise inside' is the wordplay.
'others' becomes 'rest' (I've seen this before).
'taking exercise' becomes 'train' (training is a kind of exercising. I am not sure about the 'taking' bit.).
'inside' indicates putting letters inside.
'rest' going around 'train' is 'RESTRAINT'.
Can you help me to learn more?
(Other definitions for restraint that I've seen before include "Self-discipline" , "It keeps someone or something under control" , "Curb" , "Something that holds back" , "Moderation" .)