Deer that transformed stops car in dramatic event (4,2,7)
I believe the answer is:
coup de theatre
'dramatic event' is the definition.
(term for a dramatic or sudden event)
'deer that transformed stops car' is the wordplay.
'transformed' is an anagram indicator.
'stops' indicates putting letters inside.
'car' becomes 'coupe' (coupe is a kind of car).
'deer'+'that'='deerthat'
'deerthat' anagrammed gives 'detheatr'.
'detheatr' inserted into 'coupe' is 'COUP DE THEATRE'.
'in' acts as a link.
(Other definitions for coup de theatre that I've seen before include "moment of drama" , "sensational twist" .)