Dearness allowance

It is taxable in PY 19-20 but u can claim rebate u/s 89 by filling a form 10 E before filling of ITR AY 20-21.

Why are you treating it like Arrears of salary which is taxed in the year of receipt.

Salary income is taxed on accrual or receipt basis whichever is earlier

Why not in PY18-19

Check form 16 of PY 18-19 whether Arrear payment is appearing ?

In normal course it is considered by employer also as and when paid and include accordingly in Form 16 and TDS angle too.

Better to show in AY 2020-2021 so that every where amount and TDS match.

You can take 89 relief benefit by filing 10E form. First workout under Form 10E if any benefit.

Then where does accrue or receipt apply to?

Particularly in March Salary. Salary of March as per company practice it is paid say on 7th April.

Such salary is accrued and included in Form 16 also in year ending. TDS also gets deducted in March end.

But payment happens in April next year.

1. It is correct to view that salary will be taxed based on accrual or receipt basis. But arrears of salary will be taxed only in the year in which the salary relates to. 
2. If not for arrear salary it would have been paid in the current year instead of calling it as salary arrears. 
3. As per sec 15(c), it is clearly mentioned that if a salary is in the nature of arrears (like in your case) it will be taxed in the year of receipt but relief u/s 89 will be available due difference in Income tax rates from year to year. 
4. Relief may or may not arise. But for the purpose of claiming relief from 10E must first be filed and then file your ROI. 
Please correct me if the above solution has an alternative view. 

But arrears of salary means where a retrospective increase in the basic salary is given to the employee and taxed in the year of receipt.

here, It is Dearness allowance!

1. Nowhere it is specified that arrears salary should be from basic. No definition says that "salary arrears" means only if it is in relation to basic salary. 
2. DA also forms part of your salary and if it provided as a part of arrear salary then it will also be dealt in the same way as salary arrears. 
Please correct me if the above solution has an alternative view.