Habitually silent (8)
I believe the answer is:
taciturn
'habitually silent' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for taciturn that I've seen before include "Reserved, saying little" , "withdrawn" , "Disinclined towards conversation" , "Habitually uncommunicative, saying little" , "Habitually silent" .)