Brownies regularly press skirts one by one (6)
I believe the answer is:
pixies
'brownies' is the definition.
('pixy' can be a synonym of 'brownie')
'regularly press skirts one by one' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'skirts' is an insertion indicator (skirt can mean to go around).
'one' becomes 'I' (Roman numeral).
'by one' becomes 'xi' (I can't explain this - if you can you should give a lot more credence to this answer).
The alternating letters of 'press' are 'pes'.
'i'+'xi'='ixi'
'pes' going around 'ixi' is 'PIXIES'.
(Other definitions for pixies that I've seen before include "Little (mischievous) people" , "Mischievous fairies" , "Small supernatural beings in fairy tales" , "Elves, imps" , "small fairies in children's tales" .)