Came back with a diet change for the undernourished (9)
I believe the answer is:
emaciated
'the undernourished' is the definition.
(I've seen this before)
'came back with a diet change' is the wordplay.
'back' says the letters should be written backwards.
'with' indicates putting letters inside.
'change' indicates an anagram.
'came' in reverse letter order is 'emac'.
'diet' anagrammed gives 'ited'.
'a' inserted inside 'ited' is 'iated'.
'emac'+'iated'='EMACIATED'
'for' is the link.
(Other definitions for emaciated that I've seen before include "Cadaverous" , "undernourished" , "Very thin from starvation or illness" , "of very little substance" , "Extremely thin" .)