Plain cake fraud in southeast (5)
I believe the answer is:
scone
'plain cake' is the definition.
(I've seen this before)
'fraud in southeast' is the wordplay.
'fraud' becomes 'con' (con is a kind of fraud).
'in' means one lot of letters goes inside another.
'southeast' becomes 'SE' (synonyms).
'con' inserted into 'se' is 'SCONE'.
(Other definitions for scone that I've seen before include "Savoury cake" , "Perth's settlement" , "in which a stone was once found?" , "Plain cake (for Scottish kings?)" , "Unsweetened cake" .)