Maybe an old idea slightly altered on mixed dish (6)
I believe the answer is:
rehash
'maybe an old idea slightly altered' is the definition.
I can't judge whether this definition defines the answer.
'on mixed dish' is the wordplay.
'on' becomes 're' ('on' can be mean 'with reference to').
'mixed dish' becomes 'hash' (hash is a kind of dish. I am not sure about the 'mixed' bit.).
're'+'hash'='REHASH'
Can you help me to learn more?
(Other definitions for rehash that I've seen before include "Work over (used material)" , "new disposition" , "Old material, slightly reworked and used again" , "Same material, different mix" , "Reuse (old ideas) with little improvement" .)