Soprano - safe bet to retain focus for composer (8)
I believe the answer is:
schubert
'for composer' is the definition.
(Franz Schubert, 1797-1828)
'soprano safe bet to retain focus' is the wordplay.
'soprano' becomes 's' (musical abbreviation).
'safe bet' becomes 'cert' (I've seen this before).
'to retain' means one lot of letters goes inside another.
'focus' becomes 'hub' (I've seen this before).
'cert' going around 'hub' is 'chubert'.
's'+'chubert'='SCHUBERT'
(Other definitions for schubert that I've seen before include "Lieder composer" , "Composer of song-cycle Winterreise" , "Butchers (anag) - composer, d. 1828" , "Famous man behind bars" , "Austrian composer and prolific songwriter" .)