Some activity near river (4)
I believe the answer is:
tyne
'river' is the definition.
(I've seen this before)
'some activity near' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'TYNE' is hidden within 'activity near'.
(Other definitions for tyne that I've seen before include "River of Northumberland" , "River of northeast England" , "Northumbrian river" , "Newcastle's river" , "Gateshead's river" .)