Skinless trout with an unknown sauce base (4)
I believe the answer is:
roux
'sauce base' is the definition.
(I've seen this before)
'skinless trout with an unknown' is the wordplay.
'skinless trout' becomes 'rou' (I can't explain this - if you can you should believe this answer much more).
'with' means one lot of letters go next to another.
'an unknown' becomes 'x' (algebra).
'rou'+'x'='ROUX'
Can you help me to learn more?
(Other definitions for roux that I've seen before include "Thickener for sauces" , "sauce mix" , "Mixture of fat and flour used for thickening sauces" , "Cook's thickening" , "Mixture of fat and flour used as a base for sauces" .)