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' indicates putting letters inside (some letters go outside others).
'centre' means to look at the middle letters.
The central letter of 'treatment' is 't'.
'say' going around '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" .)