Initially, happily everything you say you see in a theatre's first brief edition, was imagined (12)
I believe the answer is:
hallucinated
'brief edition was imagined' is the definition.
I can't judge whether this defines the answer.
'initially happily everything you say you see in a theatre's first' is the wordplay.
'initially' indicates taking the first letters.
'everything' becomes 'all' (all things).
'you say' shows a homophone (sound like).
'see' becomes 'C' (the word for the letter, according to Chambers).
'a theatre's first' becomes 'ated' (I can't justify this - if you can you should give a lot more credence to this answer).
The first letter of 'happily' is 'h'.
'you' sounds like 'u'.
'h'+'all'+'u'+'c'+'in'+'ated'='HALLUCINATED'
Can you help me to learn more?