One used to make yoghurt treat for consuming last month (7)
I believe the answer is:
culture
'one used to make yoghurt' is the definition.
(as in a culture of bacteria)
'treat for consuming last month' is the wordplay.
'treat' becomes 'cure' (as in medical treatment).
'for consuming' means one lot of letters goes inside another (inserted letters are consumed).
'last month' becomes 'ult' (short for 'last month' in correspondence).
'cure' enclosing 'ult' is 'CULTURE'.
(Other definitions for culture that I've seen before include "Grow (eg, bacteria, tissue cells) in artificial media" , "Attitudes and values of a society" , "Disorientation caused by a complete change environment" , "Customs of a group" , "Growth in the laboratory" .)