Question: Is it feasible for an ectoparasite such as a a Tick to create a symbiotic relationship with its host? The hosts feeds the tick and the tick has some sort of beneficial substance within its saliva. This could be some sort of naturally evolved anti-fungal or anti-biotic. This relationship would allow those with the tick to out compete those without the tick due to superior disease resistance.

Answer: 
This is literally the definition of [symbiosis](https://en.wikipedia.org/wiki/Symbiosis). The only issue in your question, as pointed out in comments, is that a parasite is definitionally not a symbiote, as a parasite derives a benefit for itself while damaging its host. Symbiotic relationships involve benefits to both organisms.

There are tons of examples of this in the world right now. Think of [remora](https://en.wikipedia.org/wiki/Remora) cleaning sharks in order to feed themselves and avoid predators, or (again already mentioned in comments) mitochondria in animal cells.

From a worldbuilding perspective, the key elements are that both organisms derive a net benefit from the relationship, and exist in such a way that they can interact to produce those benefits (like living physically close enough to interact, as in the remora, or even becoming incorporated into the same larger organism, as in the mitochondria). The net benefit piece means that there can still be negative consequences to the interactions, just that the balance must tip towards the favorable.
Take a look at anglerfish. The female is this gigantic thing, and the male is super-tiny. He bites the female and gets embedded in there. In time, his body wastes away, and he's little more than a sack of sperm for the female to use when she needs eggs fertilized. 

There are plenty of symbiotic relationships like this in the wild. Just think of something the host needs and something the symbiote needs where pairing would help both achieve their separate needs.