Strait-laced class, at Lancing originally (6)
I believe the answer is:
formal
'strait-laced' is the definition.
(thesaurus)
'class at lancing originally' is the wordplay.
'class' becomes 'form' (form class is a kind of class).
'originally' says to take the initial letters.
The first letters of 'at lancing' is 'al'.
'form'+'al'='FORMAL'
(Other definitions for formal that I've seen before include "Ceremonial or stiffly polite" , "Ceremonious or official" , "Conforming to accepted customs, say dress" , "Ceremonial or official" , "Suitable for a ceremonial occasion" .)