Milk producer cast out is glowering (8)
I believe the answer is:
scowling
'is glowering' is the definition.
(I've seen this before)
'milk producer cast out' is the wordplay.
'milk producer' becomes 'cow' (I've seen this before).
'cast' becomes 'sling' (slinging is a kind of casting).
'out' means one lot of letters goes inside another.
'cow' placed inside 'sling' is 'SCOWLING'.
(Other definitions for scowling that I've seen before include "looking unhappy?" , "Frowning with displeasure" , "Lowering" , "glowering" , "looking displeased" .)