Not altogether friendly when Arthur and I go in (9)
I believe the answer is:
partially
'not altogether' is the definition.
('partially' is the opposite to 'altogether')
'arthur and i go in' is the wordplay.
'arthur' becomes 'art' (I've seen this in other clues).
'and' says to put letters next to each other.
'go' becomes 'pally' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' indicates putting letters inside.
'art'+'i'='arti'
'arti' going within 'pally' is 'PARTIALLY'.
'friendly when' 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?
(Other definitions for partially that I've seen before include "not entirely?" , "a bit" , "to a limited extent" .)