"My friend is working at a clock company."
"A good rule of thumb is that while both particles can mark the location where a verb occurs, で de tends to be used when the verb is an action, and に ni tends to be used when the verb has more to do with just being or existing."
The site also gives the examples of「います」and 「すみます」as being verbs associated with a state of existing/being (the former literally being the verb for existing). I'm guessing in Japanese「つとめる」is considered more a state of "being", rather than an action.
How does 時計の会社 sound weird? It seems perfectly correct to me. In fact, 時計会社 seems wrong since 時計 isn't an adjective.
It may not be an adjective, but the の in this case sounds a little backwards for some reason. There's even an another example within this sentence which really confuses it. 私の友達 my friend. 時計のかいしゃ sounds like Clock's company, if it followed the same pattern. But it doesn't which is confusing.
The particle の has multiple uses. One is to show possession, as in 私の友達. Another is something like showing a category of a thing, as in 時計の会社.
Your sentence is like "about my friend in the clock company; he is working". The に particle here has to modify the verb つとめる
Isn't the に particle associated with 時計の会社 regardless? I put the same thing and am curious why it is wrong.
Possibly... or the company of clocks. No doesn't always denote possession.