Change trains leaving Italy with mum (9)
I believe the answer is:
transmute
'change' is the definition.
(transmuting is a kind of changing)
'trains leaving italy with mum' is the wordplay.
'leaving' indicates named letters should be taken away.
'italy' becomes 'i'.
'with' means one lot of letters go next to another.
'mum' becomes 'mute' (to keep mum is to remain silent).
'trains' with 'i' taken away is 'trans'.
'trans'+'mute'='TRANSMUTE'
(Other definitions for transmute that I've seen before include "Alter" , "Change the nature of (something)" , "Change eg into gold" , "published obscenity in unfinished number" .)