Opponent originally chasing tie in lead — which has no real 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')
'opponent originally chasing tie in lead which has no real' is the wordplay.
I cannot quite see how this works, but
'tie' could be 'lace' (lacing is a kind of tiing) and 'lace' is found in the answer.
'lead' could be 'pb' (Pb is the chemical symbol for lead) and 'pb' is found within the leftover letters.
'no' could be 'o' (resembles 0 - 'no' can mean 'zero') and 'o' is located in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for placebo that I've seen before include "Pill with no medicinal content" , "Calming measure" , "Ineffective substance given to humour patient" , "that has an unexplained therapeutic effect" , "patient gratified to receive such" .)