Seaman in position (6)
I believe the answer is:
rating
I believe this is a double definition.
'seaman' is the first definition.
(British term for a naval enlisted man)
'position' is the second definition.
(rating is a kind of position)
'in' is the link.
(Other definitions for rating that I've seen before include "Classification assigned to person as, say, for income" , "How many watched TV" , "General classification or naval NCO" , "Classification or NCO sailor" , "Score" .)