Prefer a violin, initially, in quartet (6)
I believe the answer is:
favour
'prefer' is the definition.
('favour' can be a synonym of 'prefer')
'a violin initially in quartet' is the wordplay.
'initially' says to take the initial letters.
'in' means one lot of letters goes inside another.
'quartet' becomes 'four' ('four' can be a synonym of 'quartet').
The first letter of 'violin' is 'v'.
'a'+'v'='av'
'av' placed within 'four' is 'FAVOUR'.
(Other definitions for favour that I've seen before include "Act of kindness or a preference." , "Go for" , "Goodwill - support" , "An act of kindness or of preferance" , "help" .)