Dashi is a soup stock made with Kombu kelp, soya sauce, mirin or sake, a touch of sugar and bonito flakes.
Any simple beef broth with a bit soya sauce and sugar is fine, not a substitute, but a close second, if you want the fish flavour, you can add a touch of Thai fish sauce, but that is not nessessary.
The kombu and bonito flakes make the distinct flavour in dashi, but can worked around.
Substitute For Dashi
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What can I use a a substitute for Dashi in Japanese cooking?