Double trouble for plant (7)
I believe the answer is:
dogbane
'plant' is the definition.
'double trouble' is the wordplay.
'double' becomes 'dog' (I can't explain this - if you can you should believe this answer much more).
'trouble' becomes 'bane' (as in a menace or trouble to someone's life).
'dog'+'bane'='DOGBANE'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for dogbane that I've seen before include "plant" , "noxious to those on leash?" .)