A line in an unspecified opera (6)
I believe the answer is:
salome
'opera' is the definition.
(I've seen this before)
'a line in an unspecified' is the wordplay.
'line' becomes 'l' (used when specifying particular lines from a poem).
'in' means one lot of letters goes inside another.
'an unspecified' becomes 'some' (some unspecified thing).
'a'+'l'='al'
'al' placed within 'some' is 'SALOME'.
(Other definitions for salome that I've seen before include "Asked for the head of John the Baptist" , "She danced famously for Herod" , "13 dn for Herod" , "Biblical dancer" , "revealing play" .)