What might be read as warning to the mob? (4,3)
I believe the answer is:
riot act
'mob?' is the definition.
Although both the answer and definition are singular nouns, I can't see how they can define each other.
I can't explain the remainder of the clue.
(Other definitions for riot act that I've seen before include "Citator may read this as stern rebuke" , "something read to little devils?" , "what constitutes warning" , "Citator of law concerning public affray" , "designed to nip trouble in the bud" .)