Still rings up, nevertheless (4,2)
I believe the answer is:
even so
'nevertheless' is the definition.
('even so' can be a synonym of 'nevertheless')
'still rings up' is the wordplay.
'still' becomes 'even' ('even' can be a synonym of 'still').
'rings' becomes 'os'.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'os' written backwards gives 'so'.
'even'+'so'='EVEN SO'
(Other definitions for even so that I've seen before include "'Nevertheless (4,2)'" , "Just the same, though that may be the case" , "Despite that, nevertheless" , "Nevertheless, albeit" , "anyway" .)