Progress at last, a substitute for milk! (8)
I believe the answer is:
screamer
'progress at last a substitute' is the definition.
I can't tell whether this definition defines the answer.
'milk' is the wordplay.
I cannot really understand how this works, but
'milk' could be 'cream' (I have seen 'Thick milk ' mean 'cream' so perhaps 'milk' could also mean 'cream') and 'cream' is found within the answer.
The remaining letters 'ser' is a valid word which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
'for' is the link.
Can you help me to learn more?
(Other definitions for screamer that I've seen before include "she'll give voice" , "powerful shot" , "Sensational newspaper headline - mere scar (anag)" , "express delivery" , "Remarkable thing; noisy person; large water bird" .)