Lost son in a shallow box (6)
I believe the answer is:
astray
'lost' is the definition.
(synonyms)
'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).
'a'+'tray'='atray'
'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)" .)