Layer cake at the edges thus (5)
I believe the answer is:
hence
'thus' is the definition.
(synonyms)
'layer cake at the edges' is the wordplay.
'layer' becomes 'hen' (hens lay eggs).
'at the edges' means to remove the middle letters.
'cake' with its middle taken out is 'ce'.
'hen'+'ce'='HENCE'
(Other definitions for hence that I've seen before include "Therefore, consequently" , "For this reason; in the future" , "As a consequence" , "Thus" , "As a result" .)