Compulsory measure for about a month (8)
I believe the answer is:
enforced
'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'.
'en'+'for'+'ced'='ENFORCED'
(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)" .)