Purple loo with poster, not the first (8)
I believe the answer is:
lavender
'purple' is the definition.
(I have seen 'Pale purple' mean 'lavender' so perhaps 'purple' could also mean 'lavender')
'loo with poster not the first' is the wordplay.
'loo' becomes 'lav' (lavatory).
'with' says to put letters next to each other.
'poster' becomes 'sender' (somebody sending or posting something).
'not the first' means to remove the first letter.
'sender' with its initial letter removed is 'ender'.
'lav'+'ender'='LAVENDER'
(Other definitions for lavender that I've seen before include "Plant prized for its scent" , "Fragrant purple flower used to scent linen etc" , "Shrub with fragrant purple flowers, used to scent linen" , "Light bluish-purple" , "Plant with aromatic mauve flowers" .)