A thing about boarding old British plane (6)
I believe the answer is:
object
'a thing' is the definition.
(I know that thing can be written as object)
'about boarding old british plane' is the wordplay.
'about' becomes 'c' (abbreviation for 'circa').
'boarding' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'plane' becomes 'jet' (jet plane).
'o'+'b'+'jet'='objet'
'c' placed within 'objet' is 'OBJECT'.
(Other definitions for object that I've seen before include "Thing; demur" , "Tangible thing" , "Express opposition; goal" , "Item - demur" , "Express dissent to the thing?" .)