Some of the quickest aircraft for part of a flight (5)
I believe the answer is:
stair
'part of a flight' is the definition.
(I've seen this in another clue)
'some of the quickest aircraft for' is the wordplay.
'some of' says the answer is hidden in the clue.
'STAIR' can be found hidden inside 'quickest aircraft for'.
(Other definitions for stair that I've seen before include "Step as part of flight" , "Bit of a flight!" , "one's seen in flight" , "that's between levels" , "Flight unit" .)