Giantess, great runner (6)
I believe the answer is:
amazon
I believe this clue is a double definition.
'giantess' is the first definition.
(as in the mythical Amazons)
'great runner' is the second definition.
(large river - something which runs)
(Other definitions for amazon that I've seen before include "Strong, aggressive woman" , "big river" , "Legendary woman warrior and river" , "Awesome female" , "Aggressive female" .)