Firmly-established intense ocean trend regularly observed (4-6)
I believe the answer is:
deep-seated
'firmly-established' is the definition.
(both can mean profound or persistent)
'intense ocean trend regularly observed' is the wordplay.
'intense' becomes 'deep' (associated in meaning).
'ocean' becomes 'sea' (synonyms).
'regularly observed' means one should take alternating letters (letters selected at regular intervals).
The alternating letters of 'trend' are 'ted'.
'deep'+'sea'+'ted'='DEEP-SEATED'
(Other definitions for deep-seated that I've seen before include "Firmly fixed" , "Firmly-established" .)