Fort Washington is on top of travel by air (4)
I believe the answer is:
waft
'travel by air' is the definition.
(I've seen this before)
'fort washington is on top' is the wordplay.
'fort' becomes 'ft' (abbreviation in place names).
'washington' becomes 'wa' (abbreviation for the US state).
'is on top' is a charade indicator (letters next to each other) (down clues only).
'ft' put after 'wa' is 'WAFT'.
'of' acts as a link.
(Other definitions for waft that I've seen before include "Float on breeze" , "Be borne on breeze" , "Sail" , "Gust, wind" , "Blow gently" .)