Thin tube inserted into a vein to deliver medication (7)
I believe the answer is:
cannula
'thin tube inserted into a vein to deliver medication' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for cannula that I've seen before include "Small tube inserted into the body to give medication" , "Small flexible tube inserted into a body for medication" , "Thin tube inserted into a vein to deliver medication" .)