Ghastly party securing state in California, America (10)
I believe the answer is:
cadaverous
'ghastly' is the definition.
(I've seen this before)
'securing state in california america' is the wordplay.
'securing' indicates putting letters inside.
'state' becomes 'aver' (both can mean to declare).
'in california' becomes 'cado' (I can't justify this - if you can you should give a lot more credence to this answer).
'america' becomes 'US' (United States).
'aver' going within 'cado' is 'cadavero'.
'cadavero'+'us'='CADAVEROUS'
'party' is the link.
This may not be right. It may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for cadaverous that I've seen before include "not looking healthy" , "Resembling a corpse" , "Corpse-like" , "Gaunt" , "Unhealthy-looking" .)