Girl taking long strides, moving fast (9)
I believe the answer is:
galloping
'moving fast' is the definition.
'girl taking long strides' is the wordplay.
'girl' becomes 'gal' ('gal' can be a synonym of 'girl').
'taking long strides' becomes 'loping'.
'gal'+'loping'='GALLOPING'
(Other definitions for galloping that I've seen before include "Running fast" , "Making haste" , "Having embarked on career" .)