Uncommunicativeness (9)
I believe the answer is:
reticence
'uncommunicativeness' is the definition.
(I know that reticence is a type of uncommunicativeness)
This is the entire clue.
(Other definitions for reticence that I've seen before include "Tendency to be reserved in communication" , "Unwillingness to talk" , "Tendency to hold back" , "'Reserve, taciturnity (9)'" , "modesty" .)