Capital gain on sale of debentures.

60,000 - 40,000(indexation is required) - STT paid = Gain / Loss.

Debentures are securities and holding period is 12 months.

Ohh sorry, debentures holding period is 3 years i.e 36 months so gain will be 60000-40000 = 20000/-

No, at the time of sale no treatment to interest will be given

No. Why do you need same?

Its clear that interest income wont be included in calculation because only sale value and cost of acquisition are included in calculation rest is taxed as and when received.

You got the right to receive the same so from the person you selling the same you can claim that interest back.