Crack down on X is a phrasal variation that means to get more strict with enforcing the rules on X.
It's not common at all but crack down without a matching on X can probably mean just being more strict in general, such as about laws, rules, etc.
It's also possible for crack down to not have this phrasal meaning and to mean "to crack, become pieces, and those pieces then fall down" which can be used figuratively.
Based on the entire paragraph from the source...
Early in 2017 things will not look auspicious, with America's
commitment to the region under President Donald Trump in doubt, and
the politics of several members lurching backwards. In Thailand the
junta that kicked out a democratic government in 2014 will dig in for
the long haul. It will go after “red-shirt” democrats and those with a
dislike for the crown prince, Vajiralongkorn, who is soon to ascend to
the throne after the death in October of his father, King Bhumibol
Adulyadej. The Malaysian prime minister, Najib Razak, immersed in
financial scandal, will crack down before a general election in 2018,
if not sooner (see box on next page). And in the Philippines (which
holds ASEANs rotating chair in 2017) under the irascible president,
Rodrigo Duterte, a vigilante crusade against drug-dealers will
threaten to suck society under.
I honestly can't tell which meaning is meant - is Najib Razak going to become more strict and "crack down" on others, or is his career going to "crack down" fall apart due to the scandal?
The writer selected a poor phrase here. Not understanding this is not your fault.