When play opens see trees close together across edge of curtain (5,5)
I believe the answer is:
first night
'together across edge of curtain' is the definition.
The definition suggests a singular noun which matches the answer.
'when play opens see trees close' is the wordplay.
I cannot quite see how this works, but
'trees' could be 'firs' (fir is a kind of tree) and 'firs' is located in the answer.
'close' could be 'nigh' (similar in meaning) and 'nigh' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for first night that I've seen before include "Start of honeymoon" , "Play's opening performance" , "to which critics regularly invited" , "(Time of) initial performance" .)