Twenty went missing on north east river (4)
I believe the answer is:
tyne
'river' is the definition.
(I've seen this before)
'twenty went missing on north east' is the wordplay.
'missing' indicates named letters should be taken away.
'on' is a charade indicator (letters next to each other).
'north east' becomes 'ne'.
'twenty' with 'went' taken out is 'ty'.
'ty'+'ne'='TYNE'
(Other definitions for tyne that I've seen before include "Newcastle's river" , "Geordie river" , "River running through Newcastle" , "River of NE England" , "Serve as evidence for" .)