It's a river, lady, and a great big one! (6)
I believe the answer is:
amazon
'it's a river' is the definition.
(Amazon is an example)
'lady and a great big one' is the wordplay.
I cannot quite see how this works, but
'and' could be 'n' (common abbreviation for 'and') and 'n' is located in the answer.
'one' could be 'a' (a thing is one thing) and 'a' is located in the answer.
The remaining letters 'mazo' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for amazon that I've seen before include "Mighty river of S America, or tough woman" , "big river" , "Mythic woman warrior" , "Strong, well-built woman" , "banker" .)