Get on with new, ultimately indecent, broker (5)
I believe the answer is:
agent
'broker' is the definition.
(synonyms)
'get on with new ultimately indecent' is the wordplay.
'get on' becomes 'age' (synonyms).
'with' means one lot of letters go next to another.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'ultimately' suggests the final letters.
The last letter of 'indecent' is 't'.
'age'+'n'+'t'='AGENT'
(Other definitions for agent that I've seen before include "One representing others" , "Performer's manager" , "Salesman" , "factor in Scotland" , "Factor, spy" .)