Following check, I'm not sure it'll keep running (6)
I believe the answer is:
stayer
'it'll keep running' is the definition.
(horse that'll run to the end of a race)
'following check i'm not sure' is the wordplay.
I cannot quite see how this works, but
'check' could be 'stay' (synonyms) and 'stay' is found in the answer.
'i'm not sure' could be 'er' (expression of hesitation) and 'er' is found in the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for stayer that I've seen before include "Horse with endurance" , "One with powers of endurance" , "One not easily giving up" , "Racehorse perhaps" , "one running over a long distance?" .)