Dictionaries say "such" is a determiner. I myself apply it this way, for instance:
I've not seen such a thing.
Dictionaries also state that 'a' is needed, but the question is why while "such" is a determiner itself?
'Such' works along with 'a' for singular objects - it is used in addition to it rather than to replace it. You would not use 'a' for 'such + [plural]', for example 'I've not seen such things'. Put simply, you use it to add information and not to replace 'a' which is doing a job to show something is in the singular.
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'Such' works along with 'a' for singular objects - it is used in addition to it rather than to replace it. You would not use 'a' for 'such + [plural]', for example 'I've not seen such things'.
Put simply, you use it to add information and not to replace 'a' which is doing a job to show something is in the singular.
Compare:
[1] I've not seen [such a thing].
[2] He had shown [such promise].
In both examples, "such" is an adjective, but its functions are slightly different. In [1] it is functioning as predeterminer (external modifier) in the bracketed noun phrase. It's external because it is outside the underlined nomin