if the legal heir not signed

Nationalised banks have rules that are in essence provided for through the government. The bank may therefore require all the surviving legal heirs to sign off on the clearance form . However, if you can obtain a letters of administration or a succession certificate from the court, then the bank may not require the other heirs to sign off. 

This is a very difficult situation. In order to get succession certificate also you will have to contend with the legal heir refusing to sign. Try to find out what exactly the refusing legal heir wants and settle the matter amicably.