Turning point in cold river, the Thames at Oxford (6)
I believe the answer is:
crisis
'turning point' is the definition.
(I've seen this before)
'cold river the thames at oxford' is the wordplay.
'cold' becomes 'c' (eg on taps).
'river' becomes 'r'.
'the thames at oxford' becomes 'isis' (I've seen this before).
'c'+'r'+'isis'='CRISIS'
'in' is the link.
(Other definitions for crisis that I've seen before include "Serious or crucial situation" , "Climax, breaking point" , "Calamity" , "Time of sever difficulty" , "crucial period" .)