Very fine (5-4)
I believe the answer is:
wafer-thin
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">very fine</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is all the clue.<br/ ></span>
(Other definitions for wafer-thin that I've seen before include "Having very little width or substance" , "Very narrow" , "Very fine" , "Warn thief stealing slim biscuit - very slim" , "Having little thickness" .)