Firm opening of Magnificat — problem for Bach? (8)
I believe the answer is:
composer
'for bach?' is the definition.
(bach is a kind of composer)
'firm opening of magnificat problem' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'opening of' says to take the initial letters.
'problem' becomes 'poser' (poser is a kind of problem).
The initial letter of 'magnificat' is 'm'.
'co'+'m'+'poser'='COMPOSER'
(Other definitions for composer that I've seen before include "he makes notes" , "A maker of music" , "Creative musician" , "Arnold, perhaps" , "Eg, Elgar" .)