Composer's toasted sandwiches have a good filling (8)
I believe the answer is:
paganini
'composer's' is the definition.
(I've seen this before)
'toasted sandwiches have a good filling' is the wordplay.
'toasted sandwiches' becomes 'panini' (panini are toasted sandwiches).
'have' indicates putting letters inside.
'good' becomes 'g' (abbreviation).
'filling' indicates putting letters inside.
'panini' enclosing 'a' is 'paanini'.
'paanini' enclosing 'g' is 'PAGANINI'.
(Other definitions for paganini that I've seen before include "who was always on the fiddle" , "Devilish performer" , "Famous Italian violinist" , "C19 violin virtuoso" , "Old bower" .)