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.
'centre' means to look at the middle letters.
The middle 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" .)