Poem concerning classical theatre (5)
I believe the answer is:
odeon
'classical theatre' is the definition.
(I've seen this before)
'poem concerning' is the wordplay.
'poem' becomes 'ode' (ode is a kind of poem).
'concerning' becomes 'on'.
'ode'+'on'='ODEON'
(Other definitions for odeon that I've seen before include "Ancient theatre - modern cinema" , "Classical Greek concert hall" , "classical theatre" , "Hal" , "Greek music theatre" .)