Clomicalm etc. (clomipramine) belongs to the group of drugs called tricyclic antidepressants (TCAs).
Clomipramine was approved for treating canine anxiety disorders by the US FDA in 2008.
TCA's work by increasing levels of serotonin and norepinephrine, two of the neurotransmitter chemicals that carry messages between brain nerve cells. Neurotransmitters like serotonin and norepinephrine are involved in the regulation of pain, pleasure, anxiety, panic, arousal, and sleep behavior (the sleep-wake cycle).