Barium and chlorine entering river causes disaster (7)
I believe the answer is:
debacle
'disaster' is the definition.
(synonyms)
'barium and chlorine entering river' is the wordplay.
'barium' becomes 'ba' (Ba is the chemical symbol for barium).
'and' says to put letters next to each other.
'chlorine' becomes 'Cl' (chemical symbol).
'entering' is an insertion indicator.
'river' becomes 'dee' (River Dee).
'ba'+'cl'='bacl'
'bacl' put within 'dee' is 'DEBACLE'.
'causes' is the link.
(Other definitions for debacle that I've seen before include "foul-up" , "Pasting" , "Lace bed (anag)" , "Catastrophe" , "great failure" .)