Container ship swallowed by very long fish (6)
I believe the answer is:
vessel
'container' is the definition.
(vessel is a kind of container)
'ship swallowed by very long fish' is the wordplay.
'ship' becomes 'ss'.
'swallowed by' means one lot of letters goes inside another.
'very' becomes 'v' (abbreviation).
'long fish' becomes 'eel' (I've seen this before).
'v'+'eel'='veel'
'ss' inserted into 'veel' is 'VESSEL'.
(Other definitions for vessel that I've seen before include "one swimming in blood?" , "Container for liquid or craft for water" , "Bark, possibly, needing water" , "Boat or bottle, say" , "Ship or large boat" .)