Follow fast runner, then slow (7)
I believe the answer is:
emulate
'follow' is the definition.
(I've seen this before)
'fast runner then slow' is the wordplay.
'fast runner' becomes 'emu' (I've seen this before).
'then' is a charade indicator (letters next to each other).
'slow' becomes 'late' (I've seen this in other clues).
'emu'+'late'='EMULATE'
(Other definitions for emulate that I've seen before include "Try to equal or surpass (someone)" , "Attempt to match" , "Try to do as well as, copy" , "Try to do as well as another" , "Try to be like" .)