In mental disorder I get sustenance (7)
I believe the answer is:
aliment
'sustenance' is the definition.
(synonyms)
'in mental disorder i' is the wordplay.
'in' means one lot of letters goes inside another.
'disorder' indicates anagramming the letters (I've seen 'diseased' mean this (I've seen this in other clues)).
'mental' with letters rearranged gives 'alment'.
'alment' enclosing 'i' is 'ALIMENT'.
'get' acts as a link.
(Other definitions for aliment that I've seen before include "Old word for food" , "Sustenance" , "Nourishment" , "Something which nourishes - meal tin (anag)" .)