Perfect; principle (5)
I believe the answer is:
ideal
I believe this clue is a double definition.
'perfect' is the first definition.
(similar in meaning)
'principle' is the second definition.
(I've seen this before)
(Other definitions for ideal that I've seen before include "Perfect but perhaps unlikely to become reality" , "Model of excellence (5)." , "Consummate" , "Model of perfection" , "Perfect; a standard of perfection" .)