Conventional (6)
'conventional' is the definition.
(synonyms)
This is the entire clue.
(Other definitions for formal that I've seen before include "Stiff - proper" , "Regular" , "Punctilious" , "Ceremonial; ordered" , "Suitable for a ceremonial occasion" .)
'conventional' is the definition.
(similar in meaning)
This is all the clue.
(Other definitions for orthodox that I've seen before include "Standard" , "not Independent-minded?" , "Conforming with tradition" , "Conservative" , "By the book" .)