Dagger mark ruins blouse (6)
I believe the answer is:
obelus
'dagger' is the definition.
(I've seen this before)
'ruins blouse' is the wordplay.
'ruins' is an anagram indicator.
'blouse' anagrammed gives 'OBELUS'.
'mark' acts as a link.
(Other definitions for obelus that I've seen before include "Dagger, in printing, to indicate a footnote" , "It indicates corruption!" , "Dagger-shaped mark" , "Dagger symbol" , "Printer's dagger" .)