The sum total of what they eat is nutritious (9)
I believe the answer is:
wholemeal
'they eat is nutritious' is the definition.
The definition suggests an adverb but the answer is not.
'the sum total of what' is the wordplay.
'the sum total' becomes 'whole' (I've seen this in another clue).
'of what' becomes 'meal' (I can't explain this - if you can you should believe this answer much more).
'whole'+'meal'='WHOLEMEAL'
Can you help me to learn more?
(Other definitions for wholemeal that I've seen before include "Like bread" , "for making bread?" , "Type of flour" , "kind of bread" , "it's so healthy" .)