Weed out on-line ad after date (9)
I believe the answer is:
dandelion
'weed' is the definition.
(plant classified as a weed)
'out on-line ad after date' is the wordplay.
'out' indicates anagramming the letters.
'after' says to put letters next to each other.
'date' becomes 'd' (common abbreviation eg in DOB, date of birth).
'on'+'line'+'ad'='onlinead'
'onlinead' is an anagram of 'andelion'.
'andelion' put after 'd' is 'DANDELION'.
(Other definitions for dandelion that I've seen before include "Weed with deeply incised leaves" , "I nod, laden with yellow flower" , "Flower with a "clock"" , "salad plant" , "One blooming" .)