One leaving prepares model to make switch (9)
I believe the answer is:
transpose
'switch' is the definition.
(to transpose two things is to switch them around)
'one leaving prepares model' is the wordplay.
'one' becomes 'i' (Roman numeral).
'leaving' indicates named letters should be taken away.
'prepares' becomes 'trains' (to train for something is to prepare for it).
'model' becomes 'pose' (model can mean to pose).
'trains' with 'i' taken out is 'trans'.
'trans'+'pose'='TRANSPOSE'
'to make' acts as a link.
(Other definitions for transpose that I've seen before include "Change position of patroness" , "Write (music) into different key" , "play in different key" , "Change to a different key" , "Cause to change places" .)