Boss renegotiated overall sum (10)
I believe the answer is:
marvellous
'boss' is the definition.
(boss can informally mean very good)
'renegotiated overall sum' is the wordplay.
'renegotiated' indicates an anagram.
'overall'+'sum'='overallsum'
'overallsum' anagrammed gives 'MARVELLOUS'.
(Other definitions for marvellous that I've seen before include "def" , "Amazing" , "Fantastic" , "inspiring wonder" , "Wonderful" .)