Fine delicate instrument in black box (6,8)
I believe the answer is:
flight recorder
'black box' is the definition.
(I've seen this before)
'fine delicate instrument' is the wordplay.
'fine' becomes 'f' (abbreviation in pencil classifications).
'delicate' becomes 'light' (similar in meaning).
'instrument' becomes 'recorder' (recorder is a kind of instrument).
'f'+'light'+'recorder'='FLIGHT RECORDER'
'in' is the link.
(Other definitions for flight recorder that I've seen before include "black box" , "may help to determine cause of accident" , "One revealing cause of crash" .)