Little-known surgeon supporting oxygen therapy (7)
I believe the answer is:
obscure
'little-known' is the definition.
(I've seen this before)
'surgeon supporting oxygen therapy' is the wordplay.
'surgeon' becomes 'bs' (Bachelor of Surgery).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'oxygen' becomes 'O' (atomic symbol for oxygen).
'therapy' becomes 'cure' (cure is a kind of therapy).
'bs' put after 'o' is 'obs'.
'obs'+'cure'='OBSCURE'
(Other definitions for obscure that I've seen before include "Indistinct or not well-known" , "Blot out" , "Not distinct" , "Dim, vague or not well known" , "Unclear - recondite" .)