Rule not starting soon (4)
I believe the answer is:
anon
'soon' is the definition.
(synonyms)
'rule not starting' is the wordplay.
'rule' becomes 'canon' (canon is a kind of rule).
'not starting' means to remove the first letter.
'canon' with its initial letter taken off is 'ANON'.
(Other definitions for anon that I've seen before include "Immediately" , "Soon, in a little while" , "Well-published writer" , "prolific author" , "Of unknown authorship" .)