Soak Britain later in seawater, island swallowed (9)
I believe the answer is:
inebriate
'soak' is the definition.
(synonyms)
'in seawater island swallowed' is the wordplay.
'seawater' becomes 'ebr' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'island' becomes 'i'.
'swallowed' becomes 'ate'.
'in'+'ebr'+'i'+'ate'='INEBRIATE'
'britain later' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
(Other definitions for inebriate that I've seen before include "Habitual drunk" , "Intoxicate" , "boozer" , "He's drunk" , "Drunkard" .)