One on flight to look hard when speaking (5)
I believe the answer is:
'one on flight' is the definition.
(part of a flight of stairs)
'look hard when speaking' is the wordplay.
'look hard' becomes 'stare' (I've seen this before).
'when speaking' shows a homophone (sound like).
'stare' sounds like 'STAIR'.
'to' is the link.
(Other definitions for stair that I've seen before include "Step as part of flight" , "A step on the way up or down" , "single step in a series" , "means to go down" , "One step in a series" .)