Food supply in desert is sustaining one, mostly (7)
I believe the answer is:
rations
'food' is the definition.
(rations is a kind of food)
'in desert is sustaining one mostly' is the wordplay.
'in desert' becomes 'rat' (abandon treacherously. I am not sure about the 'in' bit.).
'sustaining' means one lot of letters goes inside another.
'mostly' means to remove the last letter (most but not all of the word).
'one' with its final letter taken off is 'on'.
'is' going around 'on' is 'ions'.
'rat'+'ions'='RATIONS'
'supply' acts as a link.
Can you help me to learn more?
(Other definitions for rations that I've seen before include "Foodstuffs" , "Limits (supplies)" , "Fare" , "Food as issued" , "Portions of food in time of scarcity" .)