Serious: it's a final resting place (5)
I believe the answer is:
grave
I believe this is a double definition.
'serious' is the first definition.
(both can mean solemn)
'a final resting place' is the second definition.
(I've seen this in another clue)
'it's' is the link.
(Other definitions for grave that I've seen before include "Solemn, serious" , "Serious in a crypt" , "Burial place is serious" , "Sober" , "Hazardous" .)