Slow starting runner, but a cheerful one (6)
I believe the answer is:
smiler
'a cheerful one' is the definition.
(I've seen this in another clue)
'slow starting runner' is the wordplay.
'slow starting' becomes 's' (I can't explain this - if you can you should believe this answer much more).
'runner' becomes 'miler' (miler is a kind of runner).
's'+'miler'='SMILER'
'but' is the link.
Can you help me to learn more?
(Other definitions for smiler that I've seen before include "one looks quite happy" , "Happy chappie" , "Cheerful-looking character" .)