Which article should be used in this example?
Both are grammatically correct. Your choice rests upon the context in which you are using the clause.
the prime indicator of success
This implies that there is only one prime indicator of success.
a prime indicator of success
This implies that there could be more than one indicator of success that could be considered prime.
Either is acceptable given your usage.
Consider the text used to define these words:
the (article). A definite grammatical article.
a (article). The indefinite grammatical article.
When you say "a specific blah" or "a prime blah," you are using the article in a definite manner, thus
a prime are synonymous.