Prepare us a tray with last bit of cereal that's wholesome (8)
I believe the answer is:
salutary
'wholesome' is the definition.
(both can mean healthy or beneficial)
'prepare us a tray with last bit of cereal' is the wordplay.
'prepare' is an anagram indicator.
'with' means one lot of letters goes inside another.
'last bit of' says to take the final letters.
The final letter of 'cereal' is 'l'.
'us'+'a'+'tray'='usatray'
'usatray' is an anagram of 'sautary'.
'sautary' going around 'l' is 'SALUTARY'.
'that's' acts as a link.
(Other definitions for salutary that I've seen before include "Containing or bringing a timely (health) warning" , "Good for one" , "Beneficial in providing a lesson" , "Promoting good health" , "Beneficial, bringing a timely warning" .)