One on flight to watch carefully, we're told (5)
I believe the answer is:
stair
'one on flight' is the definition.
(part of a flight of stairs)
'watch carefully we're told' is the wordplay.
'watch carefully' becomes 'stare'.
'we're told' indicates a 'sounds like' (homophone) clue.
'stare' sounds like 'STAIR'.
'to' is the link.
(Other definitions for stair that I've seen before include "Tread and riser" , "Support on which to step" , "One of a 15" , "Tarsi (anag)" , "single step in a series" .)