Seaman's forbidden in the vicinity (5)
I believe the answer is:
about
'in the vicinity' is the definition.
('about' can be a synonym of 'in the vicinity')
'seaman's forbidden' is the wordplay.
'seaman' becomes 'ab'.
'forbidden' becomes 'out' ('out' can be a synonym of 'forbidden').
'ab'+'out'='ABOUT'
(Other definitions for about that I've seen before include "On the subject of, approximately" , "In the area of or in relation to" , "Apropos, concerning" , "Concerning or around" , "round" .)