Weed growing wild under logs! (9)
I believe the answer is:
groundsel
'weed' is the definition.
(I know that groundsel is a type of weed)
'growing wild under logs' is the wordplay.
'growing wild' is an anagram indicator.
'under'+'logs'='underlogs'
'underlogs' anagrammed gives 'GROUNDSEL'.
(Other definitions for groundsel that I've seen before include "plant, unwanted?" , "feed to the birds" , "Common weed" , "Plant of daisy family" .)