Subject of wartime graffiti: one line in city recalled (6)
I believe the answer is:
kilroy
'subject of wartime graffiti' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'one line in city recalled' is the wordplay.
'one' becomes 'i' (Roman numeral).
'line' becomes 'l' (used when specifying particular lines from a poem).
'in' is an insertion indicator.
'city' becomes 'york' (York is a city).
'recalled' shows that the letters should be reversed in order (to recall can mean to bring something back).
'york' backwards is 'kroy'.
'i'+'l'='il'
'il' going inside 'kroy' is 'KILROY'.
Can you help me to learn more?
(Another definition for kilroy that I've seen is " his name on walls once".)