Mariner AA refers for treatment (8)
I believe the answer is:
seafarer
'mariner' is the definition.
(I know that mariner can be written as seafarer)
'aa refers for treatment' is the wordplay.
'for treatment' is an anagram indicator (I've seen this in other clues).
'aa'+'refers'='aarefers'
'aarefers' with letters rearranged gives 'SEAFARER'.
(Other definitions for seafarer that I've seen before include "One on the water" , "Traveller over water" , "Sinbad?" , "'Mariner, sailor (8)'" , "One going by ship" .)