Six feet under here - it's serious having three feet! (9)
I believe the answer is:
graveyard
'six feet under here' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'serious having three feet' is the wordplay.
'serious' becomes 'grave' (both can mean solemn).
'having' says to put letters next to each other.
'three feet' becomes 'yard' (I've seen this before).
'grave'+'yard'='GRAVEYARD'
'it's' is the link.
Can you help me to learn more?
(Other definitions for graveyard that I've seen before include "Burial ground" , "Final destination" , "'God's acre'" , "Cemetery" , "church ground" .)