Baby milk expression (7)
I believe the answer is:
formula
I believe this clue is a double definition.
'baby milk' is the first definition.
(I know that baby milk can be written as formula)
'expression' is the second definition.
(I know that expression can be written as formula)
(Other definitions for formula that I've seen before include "Combination of ingredients or symbols" , "Set form of words or numbers" , "Group of symbols expressing a rule" , "Pattern, template" , "One that's on track" .)