Headache caused by one cereal in Middle East (8)
I believe the answer is:
migraine
'headache' is the definition.
(migraine is a kind of headache)
'one cereal in middle east' is the wordplay.
'one' becomes 'i' (Roman numeral).
'cereal' becomes 'grain' (synonyms).
'in' means one lot of letters goes inside another.
'middle east' becomes 'me' (abbreviation for Middle East).
'i'+'grain'='igrain'
'igrain' going within 'me' is 'MIGRAINE'.
'caused by' is the link.
(Other definitions for migraine that I've seen before include "Severe headache often accompanied by nausea" , "Intense headache" , "Sick headache" , "Crippling headache" , "From mare gin I get a severe headache" .)