Disaster when large taxi reverses into river (7)
I believe the answer is:
debacle
'disaster' is the definition.
(I've seen this before)
'large taxi reverses into river' is the wordplay.
'large' becomes 'l'.
'taxi' becomes 'cab' ('cab' can be a synonym of 'taxi').
'reverses' is a reversal indicator.
'into' is an insertion indicator.
'river' becomes 'dee' (River Dee).
'l'+'cab'='lcab'
'lcab' in reverse letter order is 'bacl'.
'bacl' put into 'dee' is 'DEBACLE'.
'when' is the link.
(Other definitions for debacle that I've seen before include "Sudden and violent collapse" , "Pasting" , "Utter failure" , "Humiliating disaster" , "Total disorder or defeat" .)