Little-known old remedy protecting Bachelor of Surgery (7)
I believe the answer is:
obscure
'little-known' is the definition.
(I've seen this before)
'old remedy protecting bachelor of surgery' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'remedy' becomes 'cure' (synonyms).
'protecting' indicates putting letters inside (inserted letters are held or protected).
'bachelor of surgery' becomes 'BS' (abbreviation).
'o'+'cure'='ocure'
'ocure' placed around 'bs' is 'OBSCURE'.
(Other definitions for obscure that I've seen before include "Arcane" , "Not clear, hidden" , "Hard to make out" , "Unsung" , "Dull, opaque" .)