Boatman's place: long stop (4)
I believe the answer is:
seal
'stop' is the definition.
'boatman's place long' is the wordplay.
'boatman's place' becomes 'sea'.
'long' becomes 'l'.
'sea'+'l'='SEAL'
(Other definitions for seal that I've seen before include "See 7" , "Amphibious mammal or stamp of authenticity" , "Wax stamp" , "Stamp of guarantee" , "Sea beast - authenticating mark" .)