How to create an instance with part of the methods of a class in Ruby?

You should not do this. You should instead share the problem you're solving and find a better pattern for solving it.

An example for solving this problem a little differently:

class A
  def x; end
end

module Foo
  def y; end
end

instance_with_y = A.new
instance_with_y.send :include, Foo
instance_with_y.respond_to? :y #=> true

Here is one way to solve the problem :

class X
  def a 
    11
  end
  def b
    12
  end
end

ob1 = X.new
ob1.b # => 12
ob1.singleton_class.class_eval { undef b }
ob1.b
# undefined method `b' for #<X:0x9966e60> (NoMethodError)

or, you could write as ( above and below both are same ) :

class << ob1
  undef b
end

ob1.b
# undefined method `b' for #<X:0x93a3b54> (NoMethodError)

It's possible to do what you want with ruby, as ruby can be very malleable like that, but there are much better ways. What you want to achieve seems like a really bad idea.

The problem you just described a problem inheritance is designed to solve. So really, you have two classes. Class A and also class B which inherits from class A.

class A
  def foo
    'foo'
  end
end

# B inherits all functionality from A, plus adds it's own
class B < A
  def bar
    'bar'
  end
end

# an instance of A only has the method "foo"
a = A.new
a.foo #=> 'foo'
a.bar #=> NoMethodError undefined method `bar' for #<A:0x007fdf549dee88>

# an instance of B has the methods "foo" and "bar"
b = B.new
b.foo #=> 'foo'
b.bar #=> 'bar'