Sure about medic's top antidote (5)
I believe the answer is:
serum
'antidote' is the definition.
(I've seen this in another clue)
'sure about medic's top' is the wordplay.
'about' indicates an anagram.
'top' suggests taking the first letters (in a down clue, the first letter is top).
The initial letter of 'medic' is 'm'.
'sure' anagrammed gives 'seru'.
'seru'+'m'='SERUM'
(Other definitions for serum that I've seen before include "Inoculation liquid" , "Blood component containing antibodies, used in immunisation" , "Part of blood; vaccine" , "Amber-coloured fluid in the blood" , "Watery part of blood" .)