Ancient mariner, missing sea, who can't acquire novel skills (3,3)
I believe the answer is:
old dog
'can't acquire novel skills' is the definition.
I don't know anything about this answer so I cannot tell whether this works.
'ancient mariner missing sea' is the wordplay.
'ancient' becomes 'old' (similar in meaning).
'mariner missing sea' becomes 'dog' (I am not sure about this - if you are sure you should believe this answer much more).
'old'+'dog'='OLD DOG'
'who' acts as a link.
Can you help me to learn more?
(Other definitions for old dog that I've seen before include "'You cannot teach an ... ... New tricks'" , "` `You can't teach an ... ... new tricks' '" .)