Marian upset a flier (6)
I believe the answer is:
airman
'a flier' is the definition.
('airman' can be a synonym of 'flier')
'marian upset' is the wordplay.
'upset' indicates anagramming the letters.
'marian' is an anagram of 'AIRMAN'.
(Other definitions for airman that I've seen before include "Male flyer" , "Marina (anag)" , "Aviator" , "Flier" , "He flies" .)