Only boarding school to be dated (8)
I believe the answer is:
obsolete
'dated' is the definition.
(I know that dated can be written as obsolete)
'only boarding school to be' is the wordplay.
'only' becomes 'sole' (synonyms).
'boarding' indicates putting letters inside.
'school' indicates anagramming the letters (school can mean to improve or manipulate).
'to'+'be'='tobe'
'tobe' with letters rearranged gives 'obte'.
'sole' put inside 'obte' is 'OBSOLETE'.
(Other definitions for obsolete that I've seen before include "'No longer used, out of date (8)'" , "Disused" , "Antiquated" , "No longer produced or used" , "Obese lot may be out of date" .)