Henley activity, outstanding on river (6)
I believe the answer is:
rowing
'henley activity' is the definition.
'rowing' can be an answer for 'activity' (rowing is a kind of activity). I am unsure of the 'henley' bit.
'outstanding on river' is the wordplay.
'outstanding' becomes 'owing' (an outstanding bill is still owing).
'on' means one lot of letters go next to another.
'river' becomes 'r' (abbreviation).
'owing' put after 'r' is 'ROWING'.
Can you help me to learn more?
(Other definitions for rowing that I've seen before include "Olympic event" , "Aquatic sport" , "Sculling sport?" , "Steven Redgrave's sport" , "Propelling boat with oars" .)