Constitutional decree backed by king (4)
I believe the answer is:
walk
'constitutional' is the definition.
(I've seen this in another clue)
'decree backed by king' is the wordplay.
'decree' becomes 'law' (I've seen this in another clue).
'backed' says the letters should be written in reverse.
'by' means one lot of letters go next to another.
'king' becomes 'K' (chess abbreviation).
'law' reversed gives 'wal'.
'wal'+'k'='WALK'
(Other definitions for walk that I've seen before include "Hike" , "Stretch one's legs" , "Perambulate" , "Proceed on foot" , "Be a pedestrian" .)