Missing son adopted by a criminal (6)
I believe the answer is:
absent
'missing' is the definition.
('absent' can be similar in meaning to 'missing')
'son adopted by a criminal' is the wordplay.
'son' becomes 's'.
'adopted by' indicates putting letters inside (I've seen this in other clues).
'criminal' becomes 'bent' (I've seen this before).
'a'+'bent'='abent'
's' inserted into 'abent' is 'ABSENT'.
(Other definitions for absent that I've seen before include "AWOL" , "lack of attention" , "Not present or focused" , "nowhere to be seen" , "Not present - vacuous" .)