Constantly inbound, regularly returning vessels (3-4)
I believe the answer is:
non-stop
'constantly' is the definition.
'inbound regularly returning vessels' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'returning' is a reversal indicator.
'vessels' becomes 'pots' (pot is a kind of vessel).
The alternate letters of 'inbound' are 'non'.
'pots' backwards is 'stop'.
'non'+'stop'='NON-STOP'
(Other definitions for non-stop that I've seen before include "Uninterrupted" , "Going straight from A to B" , "Continuous" , "Round-the-clock" , "Unceasing" .)