Weed that's rampant under logs (9)
I believe the answer is:
groundsel
'weed' is the definition.
(groundsel is a kind of weed)
'rampant under logs' is the wordplay.
'rampant' indicates an anagram (I've seen this in other clues).
'under'+'logs'='underlogs'
'underlogs' anagrammed gives 'GROUNDSEL'.
'that's' acts as a link.
(Other definitions for groundsel that I've seen before include "Plant of daisy family" , "feed to the birds" , "Common weed" , "plant, unwanted?" .)