Tchaikovsky's First Symphony covers exciting material (7)
I believe the answer is:
erotica
'exciting material' is the definition.
(sexually exciting material)
'tchaikovsky's first symphony covers' is the wordplay.
'tchaikovsky's first' becomes 't' (1st letter of 'tchaikovsky').
'symphony' becomes 'eroica' (Beethoven's third symphony).
'covers' means one lot of letters goes inside another.
't' placed inside 'eroica' is 'EROTICA'.
(Other definitions for erotica that I've seen before include "Sexual writings and pictures" , "Literature or art with themes of sexual love" , "works to stimulate passion?" , "Writing of a sexual nature" , "(Items of) titillating art" .)