First, let us check all vowel lengths:
tŭm vērō ĕxŏrĭtŭr clāmŏr rīpaequĕ lăcūsquĕ
A syllable with a short vowel can be long (by position).
The standard assumption is that all possible elisions happen, and that is the case here too.
There are two possibilities for a ...
The first vowel in vero is long, the second vowel of vero is elided away, and the first syllable of exoritur is long by position (because 'x' counts as two consonants since it's pronounced 'ks'). You seem to have the remainder correct. So it starts with a spondee, and all elisions occur.
It's worth noting that you can deduce from the meter that the first ...