Son having a nightmare sees skeleton (5)
I believe the answer is:
shell
'skeleton' is the definition.
(I know this)
'son having a nightmare' is the wordplay.
'son' becomes 's' (genealogical abbreviation for son).
'having' means one lot of letters go next to another.
'a nightmare' becomes 'hell'.
's'+'hell'='SHELL'
'sees' is the link.
(Other definitions for shell that I've seen before include "Carapace or cartridge" , "Hard outer case" , "attack with bombs" , "Something used instead of money" , "(Turtle's) carapace" .)