Enduring wobbly sea-legs (7)
I believe the answer is:
ageless
'enduring' is the definition.
(I've seen this before)
'wobbly sea-legs' is the wordplay.
'wobbly' indicates an anagram (I've seen this in other clues).
'sea'+'legs'='sealegs'
'sealegs' anagrammed gives 'AGELESS'.
(Other definitions for ageless that I've seen before include "Eternally young" , "Sea-legs (anag.)" , "Classic" , "Never growing old" , "not appearing to get old" .)