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 judge whether this defines the answer.
'milk' is the wordplay.
I cannot quite 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 explanation may well be incorrect...
'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" .)