Vulnerable (3,2,1,4)
I think it could be either:
out on a limb
'vulnerable' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for at risk that I've seen before include "Endangered" , "Vulnerable" , "Exposed to danger" .)
'vulnerable' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for out on a limb that I've seen before include "questionably placed" , "Alone and in danger" , "Exposed, vulnerable" , "Without support" , "Wing-walking" .)