Missing out? (6)
I believe the answer is:
absent
I believe this is a double definition.
'missing' is the first definition.
(similar in meaning)
'out?' is the second definition.
('absent' can be similar in meaning to 'out')
(Other definitions for absent that I've seen before include "AWOL" , "Missing or dreamy" , "lack of attention" , "Not where one should be" , "Not present or focused" .)