A home or treatment centre beside the sea (4)
I believe the answer is:
seat
'a home' is the definition.
(thesaurus)
'treatment centre beside the sea' is the wordplay.
'centre' indicates the central letters.
'beside' means one lot of letters go next to another (I've seen this in other clues).
The central letter of 'treatment' is 't'.
't' put after 'sea' is 'SEAT'.
'or' acts as a link.
(Other definitions for seat that I've seen before include "Chair, bench, saddle, etc" , "See 10" , "MP's place in parliament" , "Somewhere to sit" , "Spanish car" .)