Returning vessels very suddenly come to a halt (4,4)
I believe the answer is:
stop dead
'suddenly come to a halt' is the definition.
'returning vessels very' is the wordplay.
'returning' is a reversal indicator.
'vessels' becomes 'pots' (pot is a kind of vessel).
'very' becomes 'dead' (I've seen this before).
'pots' in reverse letter order is 'stop'.
'stop'+'dead'='STOP DEAD'
(Other definitions for stop dead that I've seen before include "Become immobilised" , "Pull up abruptly" .)