11ac Sums up my dissatisfaction with this crossword.
I think a case (no pun intended) can be made for both "mummers" and "mommets".
Moet(s) is a brand of champagne - "mm" could mean "I think" as in "mm .... I think the answer is ....."
Mumm(s) ditto - "er" as in "er.... I think etc."
Mommets and Mummers - both obscure I would say and both in Chambers on-line Word Wizard.
Without a definition it's impossible to know what the setter wanted. Having said that I am now leaning towards "mummers" as perhaps the less obscure of the two - pity I submitted my effort (with "mommets") days ago!