Manuel Valls promiserit se novam legem correcturum esse.
So “novam legem” is the object of “correcturum”, meaning that the sentence implies rather that he will correct the new law, rather than that he will implement new labor legislation. The difference is subtle but it is there.
tumultuantibus
Present participle plural dative of tumultuari, clearly a modern day coinage for “protesters”. Those protesting.
The grammatical gist of the entire sentence is as follows:
Non satis fuit tumultuantibus
It was not enough to the protesters. — And here you ask, What was it that was not enough? (It may be helpful to hear an unwritten [hoc]. "This was not enough to the protesters, that…)
And the answer follows
quod Manuel Valles promiserit
Here, I stand to be corrected, but I believe that in the standard concordance one would be expecting “promitteret” (imperfect subjunctive) rather than “promiserit” (perfect subjunctive). This is because the main clause has the verb in the perfect indicative, and therefore in the subordinate clause an action that occurs simultaneously to the verb goes to the imperfect subjunctive (so ‘promitteret’), and one that occurs before the verb would go to the Pluperfect subjunctive (so ‘promisisset’).
Regardless, the sense is clear. “It was not enough [to the protesters] that he promised / had promised…” (The alternative tenses reflecting the way we choose to correct ‘promitteret’.)
se novam legem correcturum esse.
Now this is clear if you remember that the verb “to promise” (promittere) is taking an infinitive clause. In this case, we are to assume that the Prime Minister said something to the effect of
“I will correct the new law.” — “[Ego] novam legem corrigam.”
How would this sentence be turned into an infinitive clause? The subject, “ego” (unnecessary in the direct speech sentence but necessary in the infinitive sentence, turns into ‘se’ in the accusative — this is because the subject of the infinitive sentence is identical to that of the verb that’s triggering it (ie, the Prime Minister is the subject both of “promiserit” and of “correcturum”); the verb in the future (corrigam) turns into the future infinitive (correcturum esse), and the rest of the sentence remains the same.