Sadist periodically traps brown bears (6)
I believe the answer is:
stands
'bears' is the definition.
('stand' can be a synonym of 'bear')
'sadist periodically traps brown' is the wordplay.
'periodically' indicates alternate letters (letters must be selected 'every so often').
'traps' indicates putting letters inside.
'brown' becomes 'tan' (tan is a kind of brown).
The alternate letters of 'sadist' are 'sds'.
'sds' placed around 'tan' is 'STANDS'.
(Other definitions for stands that I've seen before include "(Cricket) partnerships;tolerates" , "Remains upright" , "Places upright" , "Assumes an upright position" , "Endures" .)