Relief u/s 89 is allowed ?

Yes relief u/s 89 will be available on arrears pension recevied as defination of salray u/s 17(1) includes pension.

By following steps you can calculate relief on arrears received.

1. Calculate tax on total income, including the additional salary of the previous year in which the same is received;

2. Calculate the tax on total income as reduced by the additional salary of the previous year in which the same is received;

3. Calculate the difference between tax at (1) and (2). The resultant figure is tax on additional salary in the year of receipt;

4. Ascertain the previous years to which the additional salary relates;

5. Calculate the tax on the total income as increased by the relevant additional salary in respect of each such previous years and total up such taxes for all such previous years; 
6. Calculate the tax on total income without including such additional salary in respect of each such previous years and total up the taxes for all such previous years;

7. Calculate the difference between tax at (5) and (6). The resultant figure is tax on additional salary for the year to which it pertains;

8. The excess of tax computed at (3) over the tax computed at (7) is the amount of relief admissible.

If you are filling ITR by using software then as per my knowledge it has option of relief calulation on arears received, you only have to enter data in relative fields. 

Hope it will solve your query.

Thanks & Regards,

Abhi