Bird in flight on knife-edge, rising (4)
I believe the answer is:
knot
'bird' is the definition.
(knot is a kind of bird)
'in flight on knife-edge rising' is the wordplay.
'in' indicates a hidden word.
'rising' is a reversal indicator (going up in a down clue).
'tonk' is hidden within 'flight on knife edge'.
'tonk' reversed gives 'KNOT'.
(Other definitions for knot that I've seen before include "Tie; group" , "Measure of speed of, for example, frigate" , "Bird; hard part in wood" , "Tied fastening" , "Tangle; sandpiper" .)