What could have an unexpectedly beneficial effect? (7)
I believe the answer is:
placebo
'effect?' is the definition.
(I have seen 'An effect' mean 'placebo' so perhaps 'effect' could also mean 'placebo')
I can't explain the rest of the clue.
(Other definitions for placebo that I've seen before include "Medicine having no physiological effect" , "Psychosomatic remedy" , "evening 19d" , "Fake drug" , "Ineffective substance given to humour patient" .)