Fizzy tangy energy drinks unknown for long-distance runner (7)
I believe the answer is:
yangtze
'long-distance runner' is the definition.
'yangtze' can be an answer for 'runner' (Yangtze River is an example). I'm not sure about the 'long-distance' bit.
'fizzy tangy energy drinks unknown' is the wordplay.
'fizzy' indicates anagramming the letters (I've seen this in other clues).
'energy' becomes 'e' (physics symbol as in E=mc2).
'drinks' is an insertion indicator (inserted letters are consumed or drunk).
'unknown' becomes 'z' (algebra).
'tangy'+'e'='tangye'
'tangye' anagrammed gives 'yangte'.
'yangte' placed around 'z' is 'YANGTZE'.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for yangtze that I've seen before include "Asia's longest river" , "River in China; get zany (anag.)" , "Former name of China's longest river, now the Chang" , "Chinese river; get zany (anag.)" , "Longest river of Asia" .)