Lost son in a shallow box (6)
I believe the answer is:
'lost' is the definition.
'son in a shallow box' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'in' is an insertion indicator.
'shallow box' becomes 'tray' (I can't explain this - if you can you should give a lot more credence to this answer).
's' placed inside 'atray' is 'ASTRAY'.
Can you help me to learn more?
(Other definitions for astray that I've seen before include "Off one's route or lost" , "Off-course" , "A satyr (anag.)" , "Off the track - in error" , "Away from the right path like a satyr (6)" .)