Unalloyed (5)
I believe the answer is:
sheer
'unalloyed' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for sheer that I've seen before include "Almost perpendicular or very thin" , "Plumb" , "Very fine like silk stockings" , "Precipitous or fine" , "Unmitigated; perpendicular" .)