Anchor at sea in my domain (8)
I believe the answer is:
monarchy
'domain' is the definition.
(thesaurus)
'anchor at sea in my' is the wordplay.
'at sea' indicates an anagram.
'in' indicates putting letters inside.
'anchor' is an anagram of 'onarch'.
'onarch' inserted into 'my' is 'MONARCHY'.
(Other definitions for monarchy that I've seen before include "kind of government" , "Rule by a sovereign" , "realm" , "Rule by a king or queen" , "Kingship system" .)