Very, very sore unfortunately after ending on backside (4,2)
I believe the answer is:
ever so
'very' is the definition.
(I've seen this before)
'very sore unfortunately after ending on backside' is the wordplay.
'very' becomes 'v' (abbreviation).
'unfortunately' indicates anagramming the letters.
'after' says to put letters next to each other.
'ending on' indicates one should take the final letters.
The final letter of 'backside' is 'e'.
'sore' anagrammed gives 'erso'.
'v'+'erso'='verso'
'verso' put after 'e' is 'EVER SO'.
(Other definitions for ever so that I've seen before include "Intensifier" , "Jolly" , "really" , "Extremely" , "To a very great extent" .)