Hidden on top of cambium in wild trees (6)
I believe the answer is:
secret
'hidden' is the definition.
(I know that hidden can be written as secret)
'top of cambium in wild trees' is the wordplay.
'top of' says to take the initial letters.
'in' means one lot of letters goes inside another.
'wild' indicates an anagram.
The initial letter of 'cambium' is 'c'.
'trees' with letters rearranged gives 'seret'.
'c' put within 'seret' is 'SECRET'.
'on' is the link.
(Other definitions for secret that I've seen before include "Kept hidden from others" , "hushed prayer" , "Confidential (information)" , "Unknown or unobserved by others" , "It's not to be told" .)