Incorrect: There is a leaking on a pipe.
Correct: There is leaking from a pipe. OR There is a leak from a pipe.
Also correct: Water is leaking from a pipe.
We don't use 'a" with "leaking" but we would use "a" when we state that there is A LEAK
Also, You can say "ON a pipe" but it is not normally said...usually we would say leaking FROM a pipe.