Enter arrangement with friend forever (9)
I believe the answer is:
eternally
'forever' is the definition.
(I know that forever can be written as eternally)
'enter arrangement with friend' is the wordplay.
'arrangement' indicates an anagram.
'with' says to put letters next to each other.
'friend' becomes 'ally' (an ally is a friend or supporter).
'enter' with letters rearranged gives 'etern'.
'etern'+'ally'='ETERNALLY'
(Other definitions for eternally that I've seen before include "Nearly let out forever" , "all the time" , "constantly" , "for good" , "Always and ever" .)