Colouring ruined pretty blouse, at rear (3)
I believe the answer is:
dye
'colouring' is the definition.
(I've seen this before)
'ruined pretty blouse at rear' is the wordplay.
'at rear' suggests the final letters (the letter at the rear or back of the word).
The last letters of 'ruined pretty blouse' are 'DYE'.
(Other definitions for dye that I've seen before include "Colouring agent" , "Colourant" , "Colouring matter" , "Tint" , "Colouring material" .)