Like a score of ten from perfect substitute (6)
I believe the answer is:
double
This clue is a double definition.
'like a score of ten' is the first definition.
(score can mean twenty)
'perfect substitute' is the second definition.
(as in someone's double)
'from' acts as a link.
(Other definitions for double that I've seen before include "Designed for two users" , "Lookalike; twofold" , "Twice - bent over - run" , "Spitting image" , "Orwellian acceptance of conflicting principles" .)