Towards the top speed? (5)
I believe the answer is:
upper
This clue is a double definition.
'towards the top' is the first definition.
(up towards the top)
'speed?' is the second definition.
(I know that amphetamine can be written as upper)
(Other definitions for upper that I've seen before include "Higher; superior" , "Nearer the top" , "Pep pill" , "Higher in rank" , "Top of shoe; stimulant" .)