In truth, rivers do well (6)
I believe the answer is:
thrive
'do well' is the definition.
(I've seen this before)
'in truth rivers' is the wordplay.
'in' indicates a hidden word.
'THRIVE' can be found hidden inside 'truth rivers'.
(Other definitions for thrive that I've seen before include "Develop well" , "Grow strongly" , "Succeed" , "Hit rev in order to prosper" , "Grow vigorously, flourish" .)