It's fiddly but possible. (My example assumes that you want to type 'seven twelths'. Just replace my choice of numbers with the ones you require).
Press Ctrl+F9. This will create 'field braces', with your cursor in the middle.
Type EQ \F(7,12) in between the braces.
(It's important to leave a space between the 'Q' and the 'backslash')
Press Shift+F9
Your fraction should now appear but the numbers will be in the same size font as the surrounding text (which makes the fraction look too large). Highlight the fraction and reduce the font size. (If you're typing 12 point text, I suggest using 8 points for a fraction).
Chris