Weed and excreted at first in old pants (9)
I believe the answer is:
dandelion
'weed' is the definition.
(thesaurus)
'and excreted at first in old pants' is the wordplay.
'at first' indicates taking the first letters.
'pants' indicates an anagram (UK informal term for 'nonsense' or 'rubbish').
The first letter of 'excreted' is 'e'.
'and'+'e'+'in'+'old'='andeinold'
'andeinold' with letters rearranged gives 'DANDELION'.
(Other definitions for dandelion that I've seen before include "Common yellow-flowered weed" , "salad plant" , "Yellow weed" , "Flower with a "clock"" , "I nod, laden with yellow flower" .)