Missing, lacking (6)
I believe the answer is:
absent
This clue is a double definition.
'missing' is the first definition.
(similar in meaning)
'lacking' is the second definition.
(similar in meaning)
(Other definitions for absent that I've seen before include "Away" , "Not present or focused" , "lack of attention" , "Forgetful, with mind on other things" , "nowhere to be seen" .)