Rating a mariner on days at sea (8,6)
I believe the answer is:
ordinary seaman
'rating' is the definition.
('rating' can mean a non-commissioned navy sailor)
'a mariner on days at sea' is the wordplay.
'at sea' is an anagram indicator.
'a'+'mariner'+'on'+'days'='amarinerondays'
'amarinerondays' anagrammed gives 'ORDINARY SEAMAN'.
(Other definitions for ordinary seaman that I've seen before include "Lowest rank of Royal Navy sailor" , "OS in full" , "Crew member" .)