Long-distance runner called an Uber for a stretch (6)
I believe the answer is:
danube
'long-distance runner' is the definition.
'Danube' can be an answer for 'runner' (Danube River is an example). I'm not certain of the 'long-distance' bit.
'called an uber for a stretch' is the wordplay.
'DANUBE' is hidden within the letters.
But, I am uncertain how this is indicated.
Can you help me to learn more?
(Other definitions for danube that I've seen before include "5 down's river" , "River in Europe" , "Vienna's river" , "It's blue according to the song" , "Europe's musically blue river" .)