Exact constant (4)
I believe the answer is:
true
I believe this is a double definition.
'exact' is the first definition.
(I've seen this in another clue)
'constant' is the second definition.
(similar in meaning)
(Other definitions for true that I've seen before include "Exact, factual" , "to be believed?" , "Unfeigned" , "Genuine; faithful" , "Real; loyal" .)