Wartime food plans suffering a setback (4)
I believe the answer is:
spam
'wartime food' is the definition.
(tinned meat product often eaten in World War 2)
'plans suffering a setback' is the wordplay.
'plans' becomes 'maps' (I've seen this before).
'suffering a setback' says the letters should be written backwards.
'maps' reversed gives 'SPAM'.
(Other definitions for spam that I've seen before include "Tinned processed meat" , "(Send) junk mail" , "Unwanted e-mails" , "email messages" , "Tinned meat - unwanted email" .)