Constant, therefore all the same (4,2)
I believe the answer is:
even so
'all the same' is the definition.
('even so' can be a synonym of 'all the same')
'constant therefore' is the wordplay.
'constant' becomes 'even' (I've seen this before).
'therefore' becomes 'so'.
'even'+'so'='EVEN SO'
(Other definitions for even so that I've seen before include "'Nevertheless (4,2)'" , "Nevertheless, albeit" , "anyway" , "yet" , "in any event" .)