Arrest northern seaman (3)
I believe the answer is:
nab
'arrest' is the definition.
('nab' can be a synonym of 'arrest')
'northern seaman' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'seaman' becomes 'ab' (**).
'n'+'ab'='NAB'
(Other definitions for nab that I've seen before include "Take" , "Run in" , "Cop" , "Seize quickly" , "Apprehend" .)