Persistent, sure-footed, fast? Not so much! (10)
I believe the answer is:
relentless
'persistent' is the definition.
(I know that persistent can be written as relentless)
'sure-footed fast? not so much' is the wordplay.
'sure footed fast?' becomes 'relent' (I can't explain this - if you can you should give a lot more credence to this answer).
'not so much' becomes 'less' (I've seen this in another clue).
'relent'+'less'='RELENTLESS'
Can you help me to learn more?
(Other definitions for relentless that I've seen before include "sustained" , "Uncompromising" , "Allowing no respite" , "Inexorable" , "Dogged" .)