Group raised around eastern river (4)
I believe the answer is:
tees
'river' is the definition.
(River Tees )
'group raised around eastern' is the wordplay.
'group' becomes 'set' (both can mean a collection).
'raised' shows that the letters should be reversed in order (going up in a down clue).
'around' is an insertion indicator.
'eastern' becomes 'e'.
'set' written backwards gives 'tes'.
'tes' going around 'e' is 'TEES'.
(Other definitions for tees that I've seen before include "Stockton's river" , "banker" , "Golf pegs" , "Golfer's equipment" , "River on which Middlesbrough stands" .)