Is unsuited for running fast (7)
I believe the answer is:
streaks
'is' is the definition.
Although both the answer and definition are verbs in their -s form, I don't see how they can define each other.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for streaks that I've seen before include "Skaters (anag.)" , "Runs naughtily nude in public" , "Stripes for skaters" .)