Arrival at river: a few taking risks (13)
I believe the answer is:
adventuresome
'taking risks' is the definition.
(I've seen this before)
'arrival at river a few' is the wordplay.
'arrival' becomes 'advent' (advent is a kind of arrival).
'at river' becomes 'ure' (I can't explain this - if you can you should give a lot more credence to this answer).
'a few' becomes 'some' (both can mean a small amount of something).
'advent'+'ure'+'some'='ADVENTURESOME'
(Other definitions for adventuresome that I've seen before include "Rather daring arrival" , "Ready to incur risk" , "taking risks" , "Daring and enterprising" , "Ready to take risks" .)