Boatman put one out skilfully (4)
I believe the answer is:
ably
'skilfully' is the definition.
'boatman put one out' is the wordplay.
'boatman' becomes 'ab' (abbreviation for able seaman).
'put' becomes 'lay' (as in laying or putting something down).
'one' becomes 'a' (a thing is one thing).
'out' indicates named letters should be taken away.
'lay' with 'a' removed is 'ly'.
'ab'+'ly'='ABLY'
(Other definitions for ably that I've seen before include "Competently" , "In a competent fashion" , "well" , "Masterfully" , "With great skill" .)