Thickness of a hen... (5)
I believe the answer is:
layer
This clue is a double definition.
'thickness' is the first definition.
(I have seen 'Single thickness' mean 'layer' so perhaps 'thickness' could also mean 'layer')
'a hen' is the second definition.
(something which lays - eggs in this case)
'of' acts as a link.
(Other definitions for layer that I've seen before include "One of several thicknesses" , "One supplying eggs" , "Film" , "plate?" , "Source of eggs" .)