Remain, for instance, outside the treatment centre (4)
I believe the answer is:
stay
'remain' is the definition.
(synonyms)
'for instance outside the treatment centre' is the wordplay.
'for instance' becomes 'say' (both can be used to introduce an example).
'outside' means one lot of letters goes inside another (some letters go outside others).
'centre' indicates the central letters.
The centre of 'treatment' is 't'.
'say' enclosing 't' is 'STAY'.
(Other definitions for stay that I've seen before include "Rope bracing a mast etc" , "lodge" , "Rope to brace an upright" , "Remain - support" , "Stick around" .)