Sea dog at sea gets prescribed quantity of medicine (6)
I believe the answer is:
dosage
'prescribed quantity of medicine' is the definition.
(I've seen this before)
'sea dog at sea' is the wordplay.
'at sea' indicates an anagram.
'sea'+'dog'='seadog'
'seadog' with letters rearranged gives 'DOSAGE'.
'gets' is the link.
(Other definitions for dosage that I've seen before include "amount of 14 [WONDER DRUG], say" , "Medicine administered" , "precise quantity" , "Sea dog (anag)" , "Measured portion of medicine" .)