Cuckoo isn't on middle of wild plant (6)
I believe the answer is:
instil
'plant' is the definition.
(both can mean to insert or establish)
'cuckoo isn't on middle of wild' is the wordplay.
'cuckoo' indicates an anagram (I've seen this in other clues).
'on' is an insertion indicator.
'middle of' means to look at the middle letters.
The middle of 'wild' is 'il'.
'isnt' anagrammed gives 'nsti'.
'nsti' inserted inside 'il' is 'INSTIL'.
(Other definitions for instil that I've seen before include "Implant (in the mind)" , "Inculcate" , "Inject ideas into mind" , "Gradually establish something in a person's mind" , "Gradually establish (in someone's mind)" .)