Oh no! River inlet's blocked (4,2)
I believe the answer is:
dear me
'oh no' is the definition.
'river inlet's blocked' is the wordplay.
'river' becomes 'dee' (River Dee).
'inlet' becomes 'arm' (I've seen this before).
'blocked' indicates putting letters inside (to block can mean to box in or contain).
'dee' placed around 'arm' is 'DEAR ME'.
(Other definitions for dear me that I've seen before include "Mild expression of distress from expensive self" , "What a pity" , "how sad" , "Expression of mild regret from precious self" , "how distressing!" .)