Creole is both a sociohistoric and linguistic term. The fact that the linguistic attributes are difficult to put boundaries on is extremely common for linguists: we won’t even claim to tell you what the definition of “word” is! As for whether English is a Creole, it’s important to understand the motivation for Creolistics at its origin. It started, and continues, as an effort to legitimize a set of languages that bef…
The definition of a "word" is always straightforward: a word is an atomic unit of language.
However, which units are or aren't atomic varies according to what it is you're measuring.
Lexically, "catch fire" is an atomic entity, which cannot be understood as the sum of its parts. It's just one part, and it needs its own dictionary entry, separate from "catch" and from "fire".
Syntactically, "catch fire" is definitely not atomic, because the past tense is "caught fire". From this perspective, it's enough to know "catch" and "fire".
Syntactically again, we can see that "an elephant" is in variation with "two elephants" / "my elephant" / "every elephant" / etc., and it's clear that "an elephant" is not atomic, but is understood as the composition of "a(n)" with "elephant".
Phonologically, as the citation-form spelling above hinted, "an elephant" is atomic; the article cannot exist independently and must attach to another word. Without knowing what that word is, you won't know how the article is pronounced.
Specialized terms for both of these types of phenomena exist - lexical words that are too large to be syntactic words are called idioms; syntactic words that are too small to be phonological words are called clitics. But the general lesson is that, despite the definition of "word" being clear, membership in the category varies according to what aspect of the language you're looking at.