Frontal (4-2)
I believe the answer is:
head-on
'frontal' is the definition.
(synonyms)
This is all the clue.
(Other definitions for head-on that I've seen before include "(Of crash) involving the front of vehicle)" , "This collision is full frontal" , "One had (anag.)" , "Frontal, collision say" , "Of car crash, full frontal" .)