Left first three letters and a ring at a medical smock (3,4)
I believe the answer is:
lab coat
'a medical smock' is the definition.
'lab coat' can be an answer for 'smock' (I've seen this in another clue). I am unsure of the 'a medical' bit.
'left first three letters and a ring at' is the wordplay.
'left first' becomes 'l' (ast letter of 'left').
'three letters' becomes 'abc' (I can't justify this - if you can you should give a lot more credence to this answer).
'and' is a charade indicator (letters next to each other).
'a ring' becomes 'o' (letter resembling a ring).
'l'+'abc'+'o'+'at'='LAB COAT'
Can you help me to learn more?
(Another definition for lab coat that I've seen is " Scientist's protective clothing (abbr)".)