Boatman's upper crust, bleeding edge, going forward (6)
I believe the answer is:
rating
'boatman's' is the definition.
(rating is an old-fashioned term for sailor)
I can't explain the rest of the clue.
(Other definitions for rating that I've seen before include "Lowly naval rank" , "Score" , "Sailor - assessment" , "Grading or ranking" , "Angry reprimand for non-commissioned sailor?" .)