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' shows that the letters should be reversed in order.
'vessels' becomes 'pots' (pot is a kind of vessel).
The alternate letters of 'inbound' are 'non'.
'pots' reversed gives '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" .)