Mostly benefit having friend in all but name (9)
I believe the answer is:
virtually
'mostly' is the definition.
(thesaurus)
'friend in all but name' is the wordplay.
'friend in' becomes 'virtual' (I can't explain this - if you can you should believe this answer much more).
'all but' means to remove the last letter ('all but' can mean 'almost' or 'nearly').
'name' becomes 'Lyn'.
'lyn' with its last letter taken off is 'ly'.
'virtual'+'ly'='VIRTUALLY'
'benefit having' is the link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Another definition for virtually that I've seen is " In effect but not in fact".)