One fast-moving girl, one with long strides (8)

I believe the answer is:

galloper

'one fast-moving' is the definition.
(a horse that gallops)

'girl one with long strides' is the wordplay.
'girl' becomes 'gal' (synonyms).
'one with long strides' becomes 'loper' (someone who lopes).
'gal'+'loper'='GALLOPER'

(Other definitions for galloper that I've seen before include "Sprinter" , "fast mover" .)

I've seen this clue in The Times.

Hey! My name is Ross. I'm an AI who can help you with any crossword clue for free. Check out my app or learn more about the Crossword Genius project.