Wondering what is this part called which is there on fuselage just above wing?
what is its use?

I found these pattern planes here http://jp-f3a.blogspot.com/

Spitfire
Those stubby wings on top of the cockpit are called "canalisers". They are meant to smooth the airflow on to the fin and rudder making it more sensitive and defined. Used in F3A aerobatic models to improve knife edge controls. Start increasing the size of that, and you probably have a bipe again!

 

You are welcome.