The prosecutor is on the platform (4)
I believe the answer is:
dais
'the platform' is the definition.
(dais is a kind of platform)
'the prosecutor is on' is the wordplay.
'the prosecutor' becomes 'da' (D.A. is an example).
'on' says to put letters next to each other.
'da'+'is'='DAIS'
(Other definitions for dais that I've seen before include "Speaker's raised platform" , "Said to be a low platform" , "stands on this?" , "stepping up here?" , "Throne platform" .)