In entering Chinese secret society getting treatment for muscles? (6)
I believe the answer is:
toning
'treatment for muscles?' is the definition.
'in entering chinese secret society' is the wordplay.
'entering' is an insertion indicator.
'chinese secret society' becomes 'tong' (I've seen this before).
'in' placed into 'tong' is 'TONING'.
'getting' is the link.
(Other definitions for toning that I've seen before include "Conditioning" , "Hair tinting" .)