Slow to show emotion (8)
I believe the answer is:
reserved
'slow to show emotion' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for reserved that I've seen before include "Booked, being unemotional" , "Shy - set aside" , "Held back, set aside" , "Booked in advance, being somewhat taciturn?" , "Reticent; put by" .)