Lack of assertiveness (8)
I believe the answer is:
timidity
'lack of assertiveness' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for timidity that I've seen before include "State of being easily alarmed, not bold" , "Faint-heartedness" , "Lack of assertiveness" , "Shyness" , "Fearfulness" .)