"that is", "that is to say", "namely", according to the dictionary, but none of these is possible here. You will need to think in terms of what the function of δηλαδή is here, that is, can you explain what a speaker intends when they use it like this? And what in English would be likely to occur in conversation? namely would be a very unusual word in conversation, and that is is too formal here. Might it be possible to omit it entirely in some cases?
for more on δηλαδή, see here