N-PLURAL represents plural noun according to the explanation in the Collins COBUILD dictionary. Can I interpret it as a noun that is countable but unable to be used in singular form and only able to be used in plural form?
Dictionaries should usually include a usage guide that explains what things like N-PLURAL mean, but yes, your guess is basically correct. Trousers is one of the nouns in English that can really only be used in a plural form:
He wore trousers
He wore a trouser
There are a few words like this: trousers, pants, scissors, clothes, glasses (but only when it means "reading glasses" - drinking glasses can be singular). These words are technically called plurale tantum.