Spread of disease, say, after world body retreats (9)
I believe the answer is:
nutbutter
'spread' is the definition.
(I've seen this before)
'disease say after world body retreats' is the wordplay.
'disease' becomes 'T.B.' (short for tuberculosis).
'say' becomes 'utter' (to utter something is to speak or say it).
'after' means one lot of letters go next to another.
'world body' becomes 'UN' (United Nations).
'retreats' shows that the letters should be reversed in order (I've seen 'retreat' mean this).
'un' backwards is 'nu'.
'tb'+'utter'='tbutter'
'tbutter' after 'nu' is 'NUTBUTTER'.
'of' acts as a link.
(Another definition for nutbutter that I've seen is " spread".)