If i have a Hydralisk(7/6) + Queen(Ensnare) and on the opposite an 3 Zergling(4/5) with the Support Card "+1 Attack if there is an Support Unit"

so theoretically there is a Hydra with 7/7 and a Zergling with 7/5

now is the Question, because on the Ensnare Card is also written: eliminate all Support Values, does it eliminate only the +1 of the supporting Zerglings or also the +1 written on the Support Card?

Esnare affects only supporting values of units, so that whole text on Combat Cards is still valid.

As a result, you will have final values 5/5 in your example skirmish.

just remember it's called a "Reinforcment Card" and that migh eliminate the confusion.