Excellent ambition by worker perhaps to get one up (9)
I believe the answer is:
defendant
'one up' is the definition.
(someone up before a judge)
'excellent ambition by worker perhaps' is the wordplay.
'excellent' becomes 'def' (old-fashioned term for excellent).
'ambition' becomes 'end' ('end' can mean a goal or purpose).
'by' says to put letters next to each other.
'worker perhaps' becomes 'ant' (worker is a type of ant).
'def'+'end'+'ant'='DEFENDANT'
'to get' acts as a link.
(Other definitions for defendant that I've seen before include "One charged" , "The one before the court" , "Person accused" , "Against whom an action is brought in a law court" .)