Arrest northern sailor (3)
I believe the answer is:
nab
'arrest' is the definition.
(I know that arrest can be written as nab)
'northern sailor' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'sailor' becomes 'ab' (abbreviation for able seaman).
'n'+'ab'='NAB'
(Other definitions for nab that I've seen before include "Take" , "Catch (thief) (colloq.)" , "Collar (sl.)" , "Catch; grab suddenly" , "Capture, catch" .)