Medic with a certain surveillance tool (5)
I believe the answer is:
drone
'surveillance tool' is the definition.
(I've seen this in another clue)
'medic with a certain' is the wordplay.
'medic' becomes 'dr' (doctor).
'with' says to put letters next to each other.
'a certain' becomes 'one' (ie one particular example of something).
'dr'+'one'='DRONE'
(Other definitions for drone that I've seen before include "Bagpipe pipe" , "Idle person; bee" , "Talk boringly; a parasite" , "Queen's mate" , "Continuous low-pitched note" .)