How to declare interested earned from unmatured NSC (5 years) in ITR

But there is no interest entry made by the Post Office department against NSC.

Although there is no entry, the rate of interest for the series is known in advance. You can look at the India Post site or some of the tax website that will give you the accrued interest for the series. You can use this to calculate what is the accrued interest for the year.

How can I declare such earned interest per year during the NSC tenure.

One you find out the interest as above, In the ITR form you need to declare this interest as "income from other sources". You then need to add this interest to other investment under 80C and declare this as investments under 80C.

So in effect if you don't have any other investments under 80C; the additional income cancels out the benefit under 80C.