Layer (4)
'layer' is the definition.
(tier is a kind of layer)
This is all the clue.
(Other definitions for tier that I've seen before include "One of two or more rows" , "Echelon" , "Plague" , "One who secures" , "Layer, rank" .)
'layer' is the definition.
(hens lay eggs)
This is all the clue.
(Other definitions for hen that I've seen before include "Coop resident" , "Female lobster, fowl" , "Domestic fowl" , "Composed of women" , "Insufficient in amount" .)
'layer' is the definition.
(ply is a kind of layer)
This is all the clue.
(Other definitions for ply that I've seen before include "Laminate thickness" , "Work steadily as at one's trade" , "'Persist in offering, liquor say (3)'" , "Practise (a trade)" , "Use or wield, tool or weapon" .)
'layer' is the definition.
(stratum is a kind of layer)
This is all the clue.
(Other definitions for stratum that I've seen before include "stretch of rock" , "rock band" , "(Geological) layer" , "Bed of sedimentary rock" , "Social class - level" .)
'layer' is the definition.
(thesaurus)
This is all the clue.
(Other definitions for sheet that I've seen before include "Piece of linen; ship's rope" , "* [MATERIAL]" , "Piece of paper or bedding" , "Piece of paper; nautical rope" , "One of pair tucked in" .)
(Other definitions for ozone that I've seen before include "invigorating British air" , "Toxic form of oxygen" , "lively atmosphere" , "Bracing air of old, now seen as potentially toxic" , "Sea-air" .)