Hens, but not roosters (6)
I believe the answer is:
layers
'hens but not roosters' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for layers that I've seen before include "Thicknesses on top of one another" , "bands" , "fowl?" , "They lie one on top of another" , "courses" .)