Milk option (4)
'milk option' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for skim that I've seen before include "Remove top layer; read bits of" , "Remove surface layer" , "Read hastily" , "Remove (or move close to) liquid surface" , "Move lightly over a surface" .)
(Other definitions for oat that I've seen before include "Food grain" , "Cereal grass" , "Basis of porridge" , "Cereal plant" , "Cereal (singular)" .)