Decide to have new leader for plant (6)
I believe the answer is:
nettle
'decide' is the definition.
Although both the answer and definition are verbs in their base form, I can't understand how one could define the other.
'new leader for plant' is the wordplay.
'leader' suggests taking the first letters.
'for plant' becomes 'ettle' (I am not sure about this - if you are sure you should believe this answer much more).
The first letter of 'new' is 'n'.
'n'+'ettle'='NETTLE'
'to have' acts as a link.
(Other definitions for nettle that I've seen before include "it seems to flower only if dead" , "Clasp it firmly and it will not sting" , "Stinging weed - pester" , "Upset" , "anger?" .)