Group gets part without river (7)
I believe the answer is:
faction
'group' is the definition.
'part without river' is the wordplay.
'part' becomes 'fa' (fa is a kind of part).
'without river' becomes 'ction' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'fa'+'ction'='FACTION'
'gets' acts as a link.
Can you help me to learn more?
(Other definitions for faction that I've seen before include "camp" , "Dissenting group within a larger one" , "Dissenting group - drama documentary" , "Small trouble-making group within a group" , "Seeing political dissenters" .)