Fliers remain in motion (6)
I believe the answer is:
airmen
'fliers' is the definition.
('airman' can be a synonym of 'flier')
'remain in motion' is the wordplay.
'in motion' is an anagram indicator (I've seen this in other clues).
'remain' with letters rearranged gives 'AIRMEN'.
(Other definitions for airmen that I've seen before include "members of the armed forces?" , "Pilots" , "Marine (anag)" , "serving in HM forces?" , "Flyers" .)