Evaluate one with extremely plump rear (8)
I believe the answer is:
appraise
'evaluate' is the definition.
(to appraise is to evaluate or assess)
'one with extremely plump rear' is the wordplay.
'one' becomes 'a' (a thing is one thing).
'with' is a charade indicator (letters next to each other).
'extremely' means to remove the middle letters (only the extremes of the word are used).
'rear' becomes 'raise' (synonyms).
'plump' with its middle removed is 'pp'.
'a'+'pp'+'raise'='APPRAISE'
(Other definitions for appraise that I've seen before include "Assess value of (something)" , "Consider comprehensively" , "Tip off" , "Decide the value or quality of" , "Assess or evaluate" .)