Overwhelming disaster as taxi reversed left into river (7)
I believe the answer is:
debacle
'overwhelming disaster' is the definition.
(a debacle is a disaster or fiasco)
'taxi reversed left into river' is the wordplay.
'taxi' becomes 'cab' (synonyms).
'reversed' says the letters should be written backwards.
'left' becomes 'l' (common abbreviation).
'into' means one lot of letters goes inside another.
'river' becomes 'dee' (River Dee).
'cab' written backwards gives 'bac'.
'bac'+'l'='bacl'
'bacl' inserted within 'dee' is 'DEBACLE'.
'as' is the link.
(Other definitions for debacle that I've seen before include "Total disorder or defeat" , "Lace bed (anag)" , "Great disaster" , "Utter disaster" , "great failure" .)