Start walking back, as a rule (3)
I believe the answer is:
law
'a rule' is the definition.
(law is a kind of rule)
'start walking back' is the wordplay.
'start' means to remove the last letter (I've seen 'start of' mean this).
'walking' becomes 'walk' (synonyms).
'back' says the letters should be written backwards.
'walk' with its last letter taken away is 'wal'.
'wal' written backwards gives 'LAW'.
'as' is the link.
(Other definitions for law that I've seen before include "Collection of rules for a society" , "Legislation" , "Act" , "Commandment" , "Body of rules of conduct for a society" .)