Boatman taken by the force as a criminal (5)
I believe the answer is:
thief
'a criminal' is the definition.
(I know that criminal can be written as thief)
'boatman taken by the force' is the wordplay.
'boatman' becomes 'I' (where the crossword setter is 'Boatman').
'taken by' indicates putting letters inside.
'force' becomes 'f' (scientific abbreviation).
'i' placed within 'the' is 'thie'.
'thie'+'f'='THIEF'
'as' acts as a link.
(Other definitions for thief that I've seen before include "Larcenist" , "See 3" , "One who steals" , "Person who steals things" , "'Taffy was a - - '" .)