Half a dozen lines established opening scene (6)
I believe the answer is:
sestet
This might be a double definition.
'half a dozen' is the first definition.
(synonyms)
'lines established opening scene' is the second definition.
'sestet' can be an answer for 'lines' (the last six lines of a sonnet). I am not sure about the 'established opening scene' bit.
Can you help me to learn more?
(Other definitions for sestet that I've seen before include "Six together" , "musical ensemble" , "Set of six performers" , "Group of six" , "Tsetse (anag.)" .)