Poem being performed for theatre (5)
I believe the answer is:
odeon
'theatre' is the definition.
(in ancient Greece)
'poem being performed' is the wordplay.
'poem' becomes 'ode' (ode is a kind of poem).
'being performed' becomes 'on' (I've seen this in other clues).
'ode'+'on'='ODEON'
'for' is the link.
(Other definitions for odeon that I've seen before include "classical theatre" , "Place for cinema or musical and dramatic performances" , "Performances here no longer" , "music performed here" , "Ancient Greek building for music" .)