'a(n)' between adj. and noun

'Involve' throws a whole section of the armoury into disuse, though not so big a one as that threatend by 'overall'.




I've been studying English for years, but I'm still not familiar with 'a(n)' between adjective and noun because only 'a(an) + adjective + noun' seems right to me. Tough in this sentence 'so a big one' will not seem right eigher, I come across with form of 'adj. + a(n) + noun'.

Could you please explain about this??

It's generally only used in comparative constructions. You'll mainly see it combined with as, like, and such.