Vessel crossing line in swell (5)
I believe the answer is:
bloat
'swell' is the definition.
(bloating is a kind of swelling)
'vessel crossing line' is the wordplay.
'vessel' becomes 'boat' (boat is a kind of vessel**).
'crossing' is an insertion indicator.
'line' becomes 'l'.
'boat' enclosing 'l' is 'BLOAT'.
'in' is the link.
(Other definitions for bloat that I've seen before include "Puff out" , "Thus treat some fish" , "Fill up with gas" , "Cause to swell" , "Make swollen" .)