Reserve or taciturnity (9)
I believe the answer is:
reticence
I believe this clue is a double definition.
'reserve' is the first definition.
(I know that reserve can be written as reticence)
'taciturnity' is the second definition.
(synonyms)
(Other definitions for reticence that I've seen before include "Tendency to hold back" , "Restraint" , "Unwillingness to talk" , "Tendency to be reserved in communication" , "'Reserve, taciturnity (9)'" .)