The letter combos EM, ER, ERM, UM etc are often clued by phrases like "with hesitation", "I'm not sure", "showing uncertainty" etc. Erm....I'm not, er, sure why (see what I did there?) ER, of course, is also Elizabeth Regina = "Queen Elizabeth", "the late queen", and (even though she's been dead two years and the Queen is actually now Camilla) sometimes still just "the Queen" (because she was the gift that just kept giving to crossword compilers, and they have a stash of old clues they're not prepared to let go) EM and EN can also be "printer's space" or just "space"