Burns pens introduction to rhapsody for vocalists (7)
I believe the answer is:
singers
'vocalists' is the definition.
('singer' can be a synonym of 'vocalist')
'burns pens introduction to rhapsody' is the wordplay.
'burns' becomes 'singes' (singe is a kind of burn).
'pens' indicates putting letters inside.
'introduction to' says to take the initial letters (I've seen 'introductions to' mean this).
The initial letter of 'rhapsody' is 'r'.
'singes' enclosing 'r' is 'SINGERS'.
'for' acts as a link.
(Other definitions for singers that I've seen before include "Eg, choir members" , "See 26" , "Vocalists" , "eg Tenors" , "Choristers" .)