Compulsory measure for about a month (8)
I believe the answer is:
Here is my best explanation:
'compulsory' is the definition.
(I've seen this before)
'measure for about a month' is the wordplay.
'measure' becomes 'en' (en is a kind of measure**).
'about' says the letters should be written backwards.
'a month' becomes 'dec' (Dec is an example).
'dec' backwards is 'ced'.
(Other definitions for enforced that I've seen before include "Taken effect", "'Compelled observance of, a law say (8)'", "Imposed", "Caused laws to be observed", "Corn feed (anag)", "Compulsory" and "Ensured observance of rules".)