A number of googlers have been landing here over the past day or so trying to find out if there is a possessive apostrophe in ‘clothes cupboard’. Interesting!
Well, I’d say no. I think it’s safe to see ‘clothes cupboard’ as a compound noun. A compound noun is a noun made up of more than one word. There are related compound nouns such as ‘clothes peg’, ‘clothes line’ and ‘clothes horse’. Possessive apostrophes are not needed in compound nouns.
Wordwatch Towers 