Assess or estimate accepting most tender (6)
I believe the answer is:
sorest
'most tender' is the definition.
('sore' can be a synonym of 'tender')
'assess or estimate accepting' is the wordplay.
'accepting' indicates the answer is hidden within the clue (the answer has been accepted by the clue).
'SOREST' can be found hidden inside 'assess or estimate'.
(Other definitions for sorest that I've seen before include "Most aggrieved" , "Stores in most painful way" , "Most tender" , "In greatest discomfort" , "Most wounded" .)