Runs a great distance, but she's no Amazon! (4)
I believe the answer is:
lena
'runs a great distance but' is the definition.
I can't tell whether this definition defines the answer.
'she's no amazon' is the wordplay.
I cannot quite understand how this works, but
'no' could be 'n' (common abbreviation) and 'n' is found in the answer.
'she' could be 'lea' and 'lea' is present in the remaining letters.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Another definition for lena that I've seen is " River of Siberia (4400 km), the world's ninth longest".)