Will compression help defeat single letter frequency attack against a mono alphabetic substitution cipher?

Alice has a long message to send. She is using mono alphabetic substitution cipher. She thinks that if she compresses the message it may protect the text from single letter frequency attack by Eve. Does the compression help? Should she compress the message before the encryption or after the encryption? Defend.

Will compression help defeat single letter frequency attack against a mono alphabetic substitution cipher?

• Hints for these nice homework questions: There are answers valid for all common compression algorithms. A simple reason why compression is usually done before encryption is that ciphertext is usually not compressible; but does it apply in the context? Assume that the attacker knows that Alice used compression (and which) wherever she used it, per second Kerckhoffs's principle. – fgrieu Nov 1 '16 at 12:11

plain text $$\rightarrow$$ compression $$\rightarrow$$ encryption $$\rightarrow$$ cipher text