Piece of dialogue, bar changes, in script (6,1)
I believe the answer is:
linear b
'script' is the definition.
(I have seen 'Old script' mean 'linear b' so perhaps 'script' could also mean 'linear b')
'piece of dialogue bar changes in' is the wordplay.
'piece of dialogue' becomes 'line' (as in an actor's lines).
'changes' indicates anagramming the letters.
'in' indicates putting letters inside.
'bar' is an anagram of 'arb'.
'line' going within 'arb' is 'LINEAR B'.
(Other definitions for linear b that I've seen before include "Ancient writing used at Knossos" , "Early signs" , "Ancient Minoan script" , "Old script" .)