Because Spanish usually does not need subject pronouns when the subject is already clear.
Here, the subject is explicitly stated:
So saying Ella tiene un asa would sound odd unless you were contrasting it with something else. Spanish normally prefers:
instead of:
This is one of the most important points in the sentence.
But in the singular, feminine nouns beginning with a stressed a- sound often use el or un instead of la or una to make pronunciation easier.
So:
But the noun is still feminine.
You can see that from adjectives:
And in the plural, it goes back to normal feminine articles:
It is feminine.
The masculine-looking article in the singular does not make the noun masculine. It is just a sound rule used with some feminine nouns that start with a stressed a sound.
Examples of the same pattern:
Notice that the adjectives stay feminine:
In this context, asa means handle, especially the loop-like handle on things such as:
For many objects in Spanish, asa refers to the kind of handle you hold with your hand through or around.
A useful comparison:
So with jarra, asa is the natural word.
Normally, La jarra tiene un asa is the standard and most natural version.
Using un makes sense because you are saying the jug has a handle.
Without the article:
this can sound less natural on its own, or more like a label/description in certain contexts. For a normal full sentence, learners should use:
This is the normal Spanish word order:
So:
That makes it a very straightforward sentence.
Other word orders are possible in Spanish in special contexts, but for a basic statement, this is the standard pattern.
Yes, it is grammatically natural and completely understandable.
It sounds like:
In everyday speech, a native speaker might also say more specific things depending on context, but La jarra tiene un asa is perfectly normal Spanish.