Weapon dropped in river — good grief! (4,2)
I believe the answer is:
dear me
'good grief' is the definition.
I don't know anything about this answer so I can't judge whether it can be defined by this definition.
'weapon dropped in river' is the wordplay.
'weapon' becomes 'arm' (synonyms).
'dropped in' is an insertion indicator.
'river' becomes 'dee' (River Dee).
'arm' put within 'dee' is 'DEAR ME'.
Can you help me to learn more?
(Other definitions for dear me that I've seen before include "how distressing!" , "Expression of mild regret from precious self" , "Mild expression of distress from expensive self" , "Expression of dismay" , "What a pity" .)