Once hit in crash, vulnerable (2,4,3)
I believe the answer is:
on thin ice
'vulnerable' is the definition.
(I've seen this before)
'once hit in crash' is the wordplay.
'crash' indicates an anagram.
'once'+'hit'+'in'='oncehitin'
'oncehitin' is an anagram of 'ON THIN ICE'.
(Other definitions for on thin ice that I've seen before include "like a daredevil skater?" , "In a risky situation" , "At risk" , "In a dangerous situation" , "sailing close to the wind?" .)