Superficial extent of a slice of bread (4)
I believe the answer is:
area
'superficial extent' is the definition.
(I've seen this in another clue)
'a slice of bread' is the wordplay.
'slice of bread' becomes 'rea' (I can't explain this - if you can you should give a lot more credence to this answer).
'a'+'rea'='AREA'
'of' is the link.
Can you help me to learn more?
(Other definitions for area that I've seen before include "Field of activity or surface" , "District; locale" , "Extent of a 2-dimensional surface" , "part of town" , "Succession, order" .)