Abstruse remedy introduced by old surgeon (7)
I believe the answer is:
obscure
'abstruse' is the definition.
('obscure' can be a synonym of 'abstruse')
'remedy introduced by old surgeon' is the wordplay.
'remedy' becomes 'cure' ('cure' can be a synonym of 'remedy').
'introduced by' says to put letters next to each other.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'surgeon' becomes 'bs'.
'o'+'bs'='obs'
'cure' put after 'obs' is 'OBSCURE'.
(Other definitions for obscure that I've seen before include "Blot out" , "Jude may be" , "Hard to make out" , "Not clear, hidden" , "Indistinct; conceal" .)