"If blackholes collapse under there own gravity,"
They don't, but stars do.
"then what it means that gravity is a property of spacetime?"
It means that gravity is how spacetime curves, its shape, the shape of the geodesics that can be formed "on" it.
"what role the fabric of spacetime plays in contracting or collapsing the star to become a blackhole"
The acceleration necessary to keep one neutron from collapsing "through" another neutron, cannot be provided by neutron degeneracy pressure.
"or if a super massive star collapse only due to its own gravity?"
Correct.
"General relativity says gravitation is an acceleration which a body receives due to curvature of spacetime,"
Right.
"if it means there is no gravitational force but its just the distortion of spacetime?"
You got it. If you let c go to infinity, then gravitation becomes Newton's gravitational force.