Radium instrument the writer used for food (7)
I believe the answer is:
ravioli
'for food' is the definition.
(ravioli is a kind of food)
'radium instrument the writer' is the wordplay.
'radium' becomes 'Ra' (Ra is the chemical symbol for radium).
'instrument' becomes 'viol' (viol is a kind of instrument).
'the writer' becomes 'I' (the person who is writing these clues).
'ra'+'viol'+'i'='RAVIOLI'
'used' is the link.
(Other definitions for ravioli that I've seen before include "Pasta envelopes with savoury filling" , "Fare" , "Filled pasta squares" , "Filled pasta envelopes" , "Small, filled cases of dough" .)