Unbutton this and take ugly new dress off, to begin with (4)
I believe the answer is:
undo
'unbutton' is the definition.
(I know that unbutton is a more specific form of the action the undo)
'ugly new dress off to begin with' is the wordplay.
'to begin with' suggests taking the first letters.
The first letters of 'ugly new dress off' is 'UNDO'.
'this and take' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for undo that I've seen before include "Take apart" , "Spoil" , "Untie or ruin" , "Ruin or loose" , "Loosen; annul" .)