Understand serious wrongdoing but starts eating (4,2)
I believe the answer is:
digs in
'starts eating' is the definition.
(I've seen this in another clue)
'understand serious wrongdoing' is the wordplay.
'understand' becomes 'dig' (digging is a kind of understanding).
'serious wrongdoing' becomes 'sin' (sin is a kind of wrongdoing. I am not sure about the 'serious' bit.).
'dig'+'sin'='DIGS IN'
'but' acts as a link.
Can you help me to learn more?
(Another definition for digs in that I've seen is " start[S] to eat heartily".)