Careless tree-lopper might be so vulnerable! (3,2,1,4)
I believe the answer is:
out on a limb
'vulnerable' is the definition.
(I've seen this in another clue)
I cannot understand how the remainder of the clue works.
Can you help me to learn more?
(Other definitions for out on a limb that I've seen before include "Wing-walking" , "In a vulnerable position" , "Without support" , "In a precarious position" , "Alone and in danger" .)