It's very hard but laundry maids do it (4)
I believe the answer is:
iron
'it' is the definition.
The definition suggests a singular noun which matches the answer.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for iron that I've seen before include "Heavy metallic element" , "Smooth - press - club" , "uncompromising" , "Decrease" , "Heavy metal that rusts easily" .)