In truth, rivers develop and grow (6)
I believe the answer is:
thrive
'grow' is the definition.
(thriving is a kind of growing)
'in truth rivers develop' is the wordplay.
'in' says the answer is hidden in the clue.
'THRIVE' can be found hidden inside 'truth rivers develop'.
'and' acts as a link.
(Other definitions for thrive that I've seen before include "Prosper, flourish" , "Develop well" , "Prosper, grow well" , "Succeed" , "Boom" .)