Trouser is a singular word. It can be a verb (eg to trouser money) and modifies a noun (eg a trouser pocket).

Well, an interesting observation and something that I had considered when making the grid, but it doesn't change anything or make what I have written incorrect - it can only be used in the plural form when used as a noun, which is the essence of that group.

Enjoyed it. Was sure I tried secret in that group, but obvs not! Kicking myself for not seeing the solution to calm. Nice one!

Just as another observation, "scissor" can also be used in the singular as a verb

I don't know what point you're trying to make. Verbs can't be singular or plural. Scissor can be used as a verb, yes, but that's not relevant here. The relevance to this puzzle is that scissor cannot be used as a singular noun. Verbs by their nature cannot be plural or singular. Unless you're talking about conjugation which is a completely separate point.

Keep rolling?