Criminal in southeast for plain cake (5)
I believe the answer is:
scone
'plain cake' is the definition.
(I've seen this before)
'criminal in southeast' is the wordplay.
'criminal' becomes 'con' (abbreviation for 'convict').
'in' indicates putting letters inside.
'southeast' becomes 'SE' (synonyms).
'con' going into 'se' is 'SCONE'.
'for' is the link.
(Other definitions for scone that I've seen before include "Savoury cake" , "Small plain cake and Scottish town" , "Cream tea cake" , "The Stone of -----, returned to Scotland in 1996 after 700 years" , "ancient Scottish settlement" .)