Most painful -- ergo relax (6)
I believe the answer is:
sorest
'most painful' is the definition.
('sore' can be a synonym of 'painful')
'ergo relax' is the wordplay.
'ergo' becomes 'so' ('so' can be a synonym of 'ergo').
'relax' becomes 'rest' (I've seen this before).
'so'+'rest'='SOREST'
(Other definitions for sorest that I've seen before include "Most wounded" , "In greatest discomfort" , "Most tender" , "Most aggrieved" , "Stores in most painful way" .)