Potentially successful sailor breaking rank (6)
I believe the answer is:
viable
'rank' is the definition.
Both the answer and definition are adjectives. Maybe they are linked in a way I don't understand?
'potentially successful sailor breaking' is the wordplay.
'potentially successful' becomes 'vile' (I can't explain this - if you can you should believe this answer much more).
'sailor' becomes 'ab' (abbreviation for able seaman).
'breaking' is an insertion indicator.
'vile' going around 'ab' is 'VIABLE'.
Can you help me to learn more?
(Other definitions for viable that I've seen before include "Sound" , "reasonable hope of success" , "(Of plan) capable of working" , "Having a prospect of success" , "Practicable, able to be done successfully" .)