Musical instrument underneath stairs in black box (6,8)
I believe the answer is:
flight recorder
'black box' is the definition.
(I've seen this in another clue)
'musical instrument underneath stairs' is the wordplay.
'musical instrument' becomes 'recorder' (recorder is a kind of musical instrument).
'underneath' means one lot of letters go next to another (in a down clue, some letters go under others).
'stairs' becomes 'flight' (flight is a kind of stairs).
'recorder' put after 'flight' is 'FLIGHT RECORDER'.
'in' is the link.
(Other definitions for flight recorder that I've seen before include "may help to determine cause of accident" , "black box" , "One revealing cause of crash" .)