To summarise, finally both sides lost (2,3)
I believe the answer is:
in all
'to summarise' is the definition.
('in all' can introduce a summary)
'finally both sides lost' is the wordplay.
'both sides lost' says to take the centre (the letters on the sides are removed).
The middle of 'finally' is 'IN ALL'.
(Other definitions for in all that I've seen before include "Taken together" , "Summarising" , "In total" , "Taken as a whole" , "Everything considered" .)