Because of that layer, cake is hollow (5)
I believe the answer is:
hence
'because of that' is the definition.
(I've seen this in another clue)
'layer cake is hollow' is the wordplay.
'layer' becomes 'hen' (hens lay eggs).
'is hollow' means to remove the middle letters (the word is hollowed out).
'cake' with its middle removed is 'ce'.
'hen'+'ce'='HENCE'
(Other definitions for hence that I've seen before include "Thus" , "As a result" , "As a consequence" , "From this time, or for this reason" , "It follows from that fact" .)