Treat tachycardia to save associate (6)
I believe the answer is:
attach
'associate' is the definition.
(I've seen this before)
'treat tachycardia to save' is the wordplay.
'to save' says the answer is hidden in the clue (I've seen 'saving' mean this).
'ATTACH' is hidden within 'treat tachycardia'.
(Other definitions for attach that I've seen before include "Fasten - attribute" , "Add on to, couple" , "Stick" , "Join to, perhaps with staple" , "Join or seize by law" .)