In classical music, a work takes a long time (5)
I believe the answer is:
opera
'in classical music' is the definition.
'opera' can be an answer for 'music' (opera is a kind of music). I am not certain of the 'in classical' bit.
'a work takes a long time' is the wordplay.
'a work' becomes 'op' (abbreviation for opus).
'takes' means one lot of letters go next to another.
'a long time' becomes 'era' (I've seen this before).
'op'+'era'='OPERA'
Can you help me to learn more?
(Other definitions for opera that I've seen before include "Dramatic genre" , "Verdi work?" , "Drama or comedy set to music" , "Patience, for example" , ""The Magic Flute", for example" .)