Increase number within layer (8)
I believe the answer is:
heighten
'increase' is the definition.
(I know that heighten is a more specific form of the action increase)
'number within layer' is the wordplay.
'number' becomes 'eight' (eight is a kind of whole number).
'within' indicates putting letters inside.
'layer' becomes 'hen' (something which lays - eggs in this case).
'eight' put into 'hen' is 'HEIGHTEN'.
(Other definitions for heighten that I've seen before include "Intensify; make taller" , "Intensify, enhance" , "Increase in amount of degree" , "Make more intense, say awareness" , "Move up" .)