Which phrase is correct 'a roar of thunder' or 'a clap of thunder'?

It depends on which Word's connotation best fits the speaker's tone of story that the phrase is in. Use either, but be aware that they are both trite expressions. Think of a surprising [like thunder] way of saying it. How about a shock of thunder, or a grab of thunder? Or punch, fit, stun, ...

And you should consider what the phrase is actually describing - some thunder is very much like a low rumbling roar while other thunder is like a the abrupt boom of a cannon going off just overhead. ___ A clap of thunder is one single bang, a roar is longer. A grab of thunder is, with respect, a no-no. It is neither trite nor creative ... However, roar of thunderalways calls to mind the comically trite German nationalist song 'The Watch on the Rhine' which begins (in English translation): A mighty [or mighteous] roar ascends like thunder! It is pure kitsch.