Painter: knighted composer — not a writer from Italy (2,5)
I believe the answer is:
el greco
'painter' is the definition.
(nickname of Domenikos Theotokpoulos, 1541-1641)
'knighted composer not a writer from italy' is the wordplay.
'knighted composer' becomes 'elgar' (I've seen this in another clue).
'not' is a deletion indicator.
'writer from italy' becomes 'eco' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'elgar' with 'a' taken out is 'elgr'.
'elgr'+'eco'='EL GRECO'
Can you help me to learn more?
(Other definitions for el greco that I've seen before include "Spanish painter, nicknamed for his place of birth" , "He painted" , "Sp. painter, born Domen- ikos Theotokopoulos" , "Cretan-born Spanish painter, d. 1614" , "Sixteenth to seventeenth century Greek-born Spanish painter" .)