A citrus fruit having the extremes of nourishment in diet (7)
I believe the answer is:
aliment
'diet' is the definition.
(thesaurus)
'a citrus fruit having the extremes of nourishment' is the wordplay.
'citrus fruit' becomes 'lime' (lime is a kind of citrus fruit).
'having' says to put letters next to each other.
'the extremes of' says to hollow out the word (remove centre letters) (only the letters at the extremes of the word).
'nourishment' with its middle removed is 'nt'.
'a'+'lime'+'nt'='ALIMENT'
'in' is the link.
(Other definitions for aliment that I've seen before include "Old word for food" , "Sustenance" , "Something which nourishes - meal tin (anag)" , "Nourishment" .)