Weed failing to open cask with strong nuts (9)
I believe the answer is:
knotgrass
'weed' is the definition.
(I've seen this before)
'failing to open cask with strong nuts' is the wordplay.
'failing to open' means to remove the first letter (the opening letter is removed).
'with' says to put letters next to each other.
'nuts' indicates an anagram (nuts can mean crazy).
'cask' with its initial letter removed is 'ask'.
'ask' put after 'strong' is 'strongask'.
'strongask' anagrammed gives 'KNOTGRASS'.
(Other definitions for knotgrass that I've seen before include "Weed" , "plant" .)