Vanish with girl quietly so to speak (9)
I believe the answer is:
evaporate
'vanish' is the definition.
(I know that vanish can be written as evaporate)
'with girl quietly so to speak' is the wordplay.
'with' says to put letters next to each other.
'girl' becomes 'Eva' (girl's name).
'quietly' becomes 'p' (musical symbol).
'so to speak' becomes 'orate' (I can't explain this - if you can you should believe this answer much more).
'eva' next to 'p' is 'evap'.
'evap'+'orate'='EVAPORATE'
Can you help me to learn more?
(Other definitions for evaporate that I've seen before include "Disappear into thin air" , "Desiccate, dry up" , "Fade away gradually" , "Melt away" , "Turn from liquid to vapour" .)