Dogs perhaps retreating, one in flight (4)
I believe the answer is:
step
'one in flight' is the definition.
(part of a flight of stairs)
'dogs perhaps retreating' is the wordplay.
'dogs perhaps' becomes 'pets'.
'retreating' shows that the letters should be reversed in order.
'pets' written backwards gives 'STEP'.
(Other definitions for step that I've seen before include "Gradation" , "Rung, dance movement" , "Foot movement" , "Abrupt change; pace" , "Footprint" .)