Could be blooming old transatlantic vessel (9)
I believe the answer is:
mayflower
'transatlantic vessel' is the definition.
'mayflower' can be an answer for 'vessel' (I have seen 'Famous vessel' mean 'mayflower' so perhaps 'vessel' could also mean 'mayflower'). I am not certain of the 'transatlantic' bit.
'could be blooming old' is the wordplay.
'could' becomes 'may' (I've seen this before).
'be blooming old' becomes 'flower' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'may'+'flower'='MAYFLOWER'
Can you help me to learn more?
(Other definitions for mayflower that I've seen before include "Hawthorne mentioned" , "Tree" , "vessel crossing ocean" , "transatlantic ship" , "Famous vessel" .)