Lacking self-confidence (8)
I believe the answer is:
insecure
'lacking self-confidence' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for insecure that I've seen before include "Not firmly fixed" , "safe? Hardly" , "Anxious - feeling unsupported" , "Wavering" , "Fearful, unsafe." .)