Duck in flight (6)
I believe the answer is:
escape
I believe this clue is a double definition.
'duck' is the first definition.
(I've seen this before)
'flight' is the second definition.
('escape' can be a synonym of 'flight')
'in' is the link.
(Other definitions for escape that I've seen before include "Do a runner" , "has to fly" , "Make a run for it" , "'Break free, get away (6)'" , "Elude, dodge" .)