Quartet engages a violinist initially as an act of kindness (6)
I believe the answer is:
favour
'an act of kindness' is the definition.
(I've seen this before)
'quartet engages a violinist initially' is the wordplay.
'quartet' becomes 'four' (synonyms).
'engages' is an insertion indicator.
'initially' says to take the initial letters.
The initial letter of 'violinist' is 'v'.
'a'+'v'='av'
'four' placed around 'av' is 'FAVOUR'.
'as' acts as a link.
(Other definitions for favour that I've seen before include "An obliging act or kindness" , "Blessing" , "Go for" , "Something done from goodwill - prefer" , "Prefer - it's a kindness" .)