Thirst? Number tapping into big river (6)
I believe the answer is:
hunger
'thirst?' is the definition.
('hunger' can be a synonym of 'thirst')
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for hunger that I've seen before include "Strong desire - sort of strike?" , "more like a divided jury?" , "Crave" , "Longing for food" , "Craving for food" .)