Runner, one who always seems to be happy, given no start (5)

I believe the answer is:

miler

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'runner' is the definition.
(miler is a kind of runner)

I cannot understand how the remainder of the clue works.

(Other definitions for miler that I've seen before include "one who competes in the long run" , "Runner over a specific distance" , "Pre-metric middle-distance runner" , "horse" , "Athlete running 1,760 yards" .)

I've seen this clue in the Sydney Morning Herald.

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