Work taking a long time — Parsifal, perhaps (5)
I believe the answer is:
opera
'parsifal perhaps' is the definition.
(I know that Parsifal is a opera)
'work taking a long time' is the wordplay.
'work' becomes 'op' (short for opus).
'taking' means one lot of letters go next to another.
'a long time' becomes 'era' (a long period of time).
'op'+'era'='OPERA'
(Other definitions for opera that I've seen before include "eg Peter Grimes" , "La Scala production, perhaps" , "One at the Met" , "Dramatic genre" , "Fidelio for instance" .)