Large taxi overturned in river disaster (7)
I believe the answer is:
debacle
'disaster' is the definition.
(synonyms)
'large taxi overturned in river' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'taxi' becomes 'cab' (synonyms).
'overturned' shows that the letters should be reversed in order.
'in' means one lot of letters goes inside another.
'river' becomes 'Dee' (River Dee).
'l'+'cab'='lcab'
'lcab' back-to-front is 'bacl'.
'bacl' going inside 'dee' is 'DEBACLE'.
(Other definitions for debacle that I've seen before include "A complete collapse or messy failure" , "Rout; disaster" , "Sudden and ignominious failure" , "Utter disaster" , "Lace bed (anag)" .)