Came back with a diet designed 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 designed' is the wordplay.
'back' says the letters should be written backwards.
'with' means one lot of letters goes inside another.
'designed' indicates an anagram (designed with the letters).
'came' written backwards gives 'emac'.
'diet' anagrammed gives 'ited'.
'a' going into 'ited' is 'iated'.
'emac'+'iated'='EMACIATED'
'for' is the link.
(Other definitions for emaciated that I've seen before include "Abnormally thin and weak" , "Very thin from starvation or illness" , "Thin from illness or hunger" , "Cadaverous" , "wasted" .)