Girl with speed to cross river and vanish (9)
I believe the answer is:
evaporate
'vanish' is the definition.
(I've seen this in another clue)
'girl with speed to cross river' is the wordplay.
'girl' becomes 'eva' ().
'with' says to put letters next to each other.
'speed' becomes 'rate' (the speed at which something happens).
'to cross' is an insertion indicator.
'river' becomes 'Po' (Italian river).
'eva'+'rate'='evarate'
'evarate' going around 'po' is 'EVAPORATE'.
'and' is the link.
(Other definitions for evaporate that I've seen before include "Turn liquid to gas" , "Desiccate, dry up" , "go a stage further?" , "let off steam" , "Melt away" .)