Fiasco as large taxi reversed into river (7)
I believe the answer is:
debacle
'fiasco' is the definition.
(synonyms)
'large taxi reversed into river' is the wordplay.
'large' becomes 'L' (eg in clothes sizes).
'taxi' becomes 'cab' (synonyms).
'reversed' shows that the letters should be reversed in order.
'into' means one lot of letters goes inside another.
'river' becomes 'Dee' (River Dee).
'l'+'cab'='lcab'
'lcab' backwards is 'bacl'.
'bacl' put inside 'dee' is 'DEBACLE'.
'as' is the link.
(Other definitions for debacle that I've seen before include "Utter failure" , "A sound defeat" , "Pasting" , "Disatrous collapse" , "Rout; disaster" .)