Eternal problem of unstable sea legs (7)
I believe the answer is:
ageless
'eternal' is the definition.
(I know that eternal can be written as ageless)
'unstable sea legs' is the wordplay.
'unstable' indicates an anagram.
'sea'+'legs'='sealegs'
'sealegs' anagrammed gives 'AGELESS'.
'problem of' is the link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for ageless that I've seen before include "Forever young" , "Never growing old" , "Lasting forever" , "Never appearing to grow old" , "Always youthful" .)