Very fine biscuit with no excess fat (5-4)
I believe the answer is:
wafer-thin
'very fine' is the definition.
(I've seen this before)
'biscuit with no excess fat' is the wordplay.
'biscuit' becomes 'wafer' (wafer is a kind of biscuit).
'with no excess fat' becomes 'thin' (I can't justify this - if you can you should believe this answer much more).
'wafer'+'thin'='WAFER-THIN'
Can you help me to learn more?
(Other definitions for wafer-thin that I've seen before include "Very fine" , "Having very little width or substance" , "Warn thief stealing slim biscuit - very slim" , "Having little thickness" , "Very narrow" .)