Layer cake unfilled for this reason (5)
I believe the answer is:
hence
'for this reason' is the definition.
(I've seen this before)
'layer cake unfilled' is the wordplay.
'layer' becomes 'hen' (something which lays - eggs in this case).
'unfilled' means to remove the middle letters.
'cake' with its centre removed is 'ce'.
'hen'+'ce'='HENCE'
(Other definitions for hence that I've seen before include "From this place or time" , "Therefore, consequently" , "It follows from that fact" , "Thus" , "As a consequence" .)