Wild sea coast gripping one’s companion (9)
I believe the answer is:
associate
'companion' is the definition.
(I know that companion can be written as associate)
'wild sea coast gripping one's' is the wordplay.
'wild' indicates an anagram.
'gripping' means one lot of letters goes inside another (inserted letters are gripped).
'one' becomes 'i' (Roman numeral).
'sea'+'coast'='seacoast'
'seacoast' is an anagram of 'assocate'.
'assocate' enclosing 'i' is 'ASSOCIATE'.
(Other definitions for associate that I've seen before include "Connect; partner" , "Not having full status" , "Partner in work or business" , "Make a logical or causal connection" , "Join" .)