Son: nightmare case (5)
I believe the answer is:
shell
'case' is the definition.
(I know that case can be written as shell)
'son nightmare' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'nightmare' becomes 'hell'.
's'+'hell'='SHELL'
(Other definitions for shell that I've seen before include "Protective outer case" , "See 18" , "pepper?" , "Hard casing; a racing boat" , "Explosive projectile" .)