Fail to stay upright, being voluble in ocean, swimming (11)
I believe the answer is:
overbalance
'fail to stay upright' is the definition.
(lose one's balance)
'voluble in ocean swimming' is the wordplay.
'voluble' becomes 'verbal' (I can't explain this - if you can you should give a lot more credence to this answer).
'in' means one lot of letters goes inside another.
'swimming' indicates an anagram (letters swim around into a new order).
'ocean' anagrammed gives 'oance'.
'verbal' inserted within 'oance' is 'OVERBALANCE'.
'being' is the link.
Can you help me to learn more?
(Other definitions for overbalance that I've seen before include "destabilise" , ""take a tumble."" , "Topple" , "Tip" , "Capsize" .)