Very fine (5-4)
I believe the answer is:
wafer-thin
'very fine' is the definition.
(I've seen this before)
This is all the clue.
(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" .)