Possible Clues:

• Plump single parent is best
• Plump mother protecting one best
• Choose one mother like this; it's all for the best
• Most favourable (conditions)
• It's all for the best for one of 12 across at last (7)
• Pick one parent - the most favourable
• Model one found in plump old lady
• Work to get it up to Mother, it's all for the best (7)
• It's best one's embraced by plump mother
• 'The problem of — wealth distribution . . . will not yield to the well-intentioned Utopian amateurs' (Shaw, Farfetched Fables)