Plant one between lines at rear of nursery (4)
I believe the answer is:
lily
'plant' is the definition.
The answer is an example.
'one between lines at rear of nursery' is the wordplay.
'one' becomes 'I' (Roman numeral).
'between' indicates putting letters inside.
'lines' becomes 'll' (an abbreviation for line written twice).
'at rear of' suggests the final letters (I've seen 'at rear' mean this (the letter at the rear or back of the word)).
The final letter of 'nursery' is 'y'.
'i' going inside 'll' is 'lil'.
'lil'+'y'='LILY'
(Other definitions for lily that I've seen before include "Flower -- girl" , "Type of flower" , "Eg, arum, calla" , "Bulbous plant with trumpet-shaped flowers" , "Flower of purity" .)