Returning thanks to family, first son devised meal plan that's high in fat and controversial (6,4)
I believe the answer is:
atkins diet
'meal plan that's high in fat and controversial' is the definition.
I don't know anything about this answer so I cannot tell whether this works.
'returning thanks to family first son devised' is the wordplay.
'returning' is a reversal indicator.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'to family' becomes 'kin' (synonyms).
'first son' becomes 's' (dst letter of 'son').
'devised' becomes 'diet' (I can't justify this - if you can you should give a lot more credence to this answer).
'ta' back-to-front is 'at'.
'at'+'kin'+'s'+'diet'='ATKINS DIET'
Can you help me to learn more?
(Other definitions for atkins diet that I've seen before include "A high-protein, high-fat, low-carb regimen" , "A low-carbohydrate regime" .)