Missing son taken in by a criminal (6)
I believe the answer is:
absent
'missing' is the definition.
('absent' can be similar in meaning to 'missing')
'son taken in by a criminal' is the wordplay.
'son' becomes 's'.
'taken in by' means one lot of letters goes inside another.
'criminal' becomes 'bent' (I've seen this before).
'a'+'bent'='abent'
's' placed inside 'abent' is 'ABSENT'.
(Other definitions for absent that I've seen before include "Missing, lacking" , "lack of attention" , "Not present or focused" , "elsewhere" , "Missing or dreamy" .)