Desert contains nutrients? Really! (3,8,4)
I believe the answer is:
for goodness sake
'really' is the definition.
(I've seen this before)
'desert contains nutrients?' is the wordplay.
'desert' becomes 'forsake' (synonyms).
'contains' means one lot of letters goes inside another.
'nutrients?' becomes 'goodness' (goodness in food).
'forsake' going around 'goodness' is 'FOR GOODNESS SAKE'.
(Other definitions for for goodness sake that I've seen before include "Words of exasperation" , "please!" , "Really!" .)