It looks like Clomicalm is a tricyclic antidepressant used to treat separation anxiety in dogs. Here is the page with all of the uses, warnings, drug interactions, etc:
Clomicalm Official FDA information, side effects and uses.
The major side effects are emesis (vomiting; in 20% of treated dogs) and lethargy (tiredness; in 14% of treated dogs). Less common side effects include diarrhea, thirstiness, decreased appetite, etc.
It is metabolized by the liver and interacts with other drugs and anesthesia, so be especially cautious if your dog is receiving other medications, or before surgery, or if it has liver problems. Since it's a tricyclic antidepressant, it may take a couple weeks before it has noticeable effects--this is a drug that you give your dog long term.
Here is some critical information from the link I gave above:
The specific methods of behavioral modification used in clinical trials involved desensitization and counterconditioning techniques. Since the manifestation of separation anxiety can vary according to the individual dog, it is advised that a specific behavior modification plan be developed based on a professional assessment of each individual case. Once the desired clinical effect is achieved and the owners have successfully instituted the appropriate behavioral modification, the dose of Clomicalm Tablets may be reduced to maintain the desired effect or discontinued. Withdrawal side effects were not reported in studies with Clomicalm Tablets in dogs. However, in clinical practice, it is recommended to taper the individual patient dose while continuing to monitor the dog’s behavior and clinical status through the dose reduction or withdrawal period. Continued behavioral modification is recommended to prevent recurrence of the clinical signs.
In other words, besides giving this medicine, you should try to desensitize your dog to the things it fears or which cause anxiety. Once the anxiety is controlled, the dose of medicine can be reduced or stopped. There are no withdrawal symptoms, but it is recommended that if you're going to stop the medication, you taper it off gradually.
Bottom line: If it were my dog, and the anxiety were extreme, I'd rather give this medication than Xanax (which is more toxic and does have withdrawal symptoms) PROVIDED that my dog were otherwise healthy and had no liver issues. I trust that your vet ran the necessary tests. Good luck, let us know what you decide to do, and if you do put him on Clomicalm, let us know how it works out!