Stop migrants camping here — not one is left (4,2,4)
I believe the answer is:
port of call
'stop migrants camping here not' is the definition.
The answer is a location as well as being a singular noun. This is suggested by the definition.
'one is left' is the wordplay.
I cannot quite see how this works, but
'one' could be 'a' and 'a' is present in the answer.
'left' could be 'port' (left side of a ship - opposite of starboard) and 'port' is present in the answer.
This may be the basis of clue (or it may be nonsense).
(Other definitions for port of call that I've seen before include "place to stop en route" , "Place briefly visited" .)