Volcanic flow starts to bury oceanic basin (6)
I believe the answer is:
lavabo
'basin' is the definition.
(lavabo is a kind of basin)
'volcanic flow starts to bury oceanic' is the wordplay.
'volcanic flow' becomes 'lava' (I've seen this in another clue).
'starts to' indicates taking the first letters.
The first letters of 'bury oceanic' is 'bo'.
'lava'+'bo'='LAVABO'
(Other definitions for lavabo that I've seen before include "Washing trough" , "bowl" , "bathroom fitting" , "Catholic washing" , "basin" .)