Not standing behind the sea, that's firmly established (4,6)
I believe the answer is:
deep seated
'firmly established' is the definition.
(I've seen this before)
'not standing behind the sea' is the wordplay.
'not standing' becomes 'seated' ('seated' is the opposite to 'standing').
'behind' means one lot of letters go next to another.
'the sea' becomes 'deep' (I can't justify this - if you can you should believe this answer much more).
'seated' put after 'deep' is 'DEEP-SEATED'.
'that's' is the link.
Can you help me to learn more?
(Other definitions for deep seated that I've seen before include "Fixed" , "like luxury armchairs?" , "Firmly established" , "like Neptune on his throne?" , "well down in level" .)