Bard potentially starts to redefine theatre with Macbeth's opening lines (8)
I believe the answer is:
minstrel
'bard' is the definition.
(thesaurus)
'potentially starts to redefine theatre with macbeth's opening lines' is the wordplay.
'potentially' indicates an anagram (letters have the potential to spell a new word).
'starts to' suggests taking the first letters.
'with' means one lot of letters go next to another.
'opening' suggests taking the first letters.
The initial letters of 'redefine theatre' is 'rt'.
The initial letter of 'macbeths' is 'm'.
'm'+'lines'='mlines'
'rt' put after 'mlines' is 'mlinesrt'.
'mlinesrt' anagrammed gives 'MINSTREL'.
(Other definitions for minstrel that I've seen before include "one entertains" , "Travelling singer in Middle Ages" , "Old song 'The ... boy to the wars has gone'" , "Travelling troubadour of old" , "Jongleur" .)