A duke boarding flight for a lark (8)
I believe the answer is:
escapade
'a lark' is the definition.
('escapade' can be a synonym of 'lark')
'a duke boarding flight' is the wordplay.
'duke' becomes 'd'.
'boarding' means one lot of letters goes inside another.
'flight' becomes 'escape' ('escape' can be a synonym of 'flight'**).
'a'+'d'='ad'
'ad' put within 'escape' is 'ESCAPADE'.
'for' is the link.
(Other definitions for escapade that I've seen before include "Daring stunt" , "Mischievous exploit" , "Daring adventure" , "quite a lark" , "Adventurous incident" .)