Son rejected flat (8)
I believe the answer is:
squashed
'flat' is the definition.
(thesaurus)
'son rejected' is the wordplay.
'son' becomes 's'.
'rejected' becomes 'quashed'.
's'+'quashed'='SQUASHED'
(Other definitions for squashed that I've seen before include "pressed in" , "Flattened or crushed" , "humiliated" .)