Berlioz finally appears in catalogue as composer (5)
I believe the answer is:
liszt
'composer' is the definition.
(Franz Liszt)
'berlioz finally appears in catalogue' is the wordplay.
'finally' indicates one should take the final letters.
'appears in' means one lot of letters goes inside another.
'catalogue' becomes 'list' (a catalogue is a list of items).
The last letter of 'berlioz' is 'z'.
'z' going into 'list' is 'LISZT'.
'as' is the link.
(Other definitions for liszt that I've seen before include "Pianist and composer Cockneys associate with Brahms" , "music writer" , "Hungarian composer,d. 1886" , "Composer of the Hungarian Rhapsodies" , "Franz . . . . ., great Hungarian composer" .)